Sweet potato gratin almost always fails because of the knife and the oven, not the recipe. Cut the tubers into slices 5 or 6 mm thick, and after 55 minutes you pull a gratin from the oven that looks golden on top and is still fibrous and chewy underneath. We stick firmly to 3 mm, and not a millimetre more.
We have been making this gratin for years as a vegetarian main for four, sometimes as a side dish with salmon fillet or chicken breast. The 700 g of sweet potatoes, the red pepper and the 300 g of mushrooms disappear into the crème fraîche, double cream and feta to form a sauce that thickens to a custard-like consistency as the three eggs set in the oven. We never need a roux here. The binding comes from the eggs, the flavour from the cheese, and the seasoning from the ginger, garlic and oregano.
Sweet Potato Gratin with the Thermomix®
Ingredients 0 / 15 ✓
- 700 g sweet potato
- 1 red pepper
- 300 g chestnut mushrooms
- 50 g Gouda
- 1 piece ginger 2 x 2 cm
- 1 garlic clove
- 10 g butter
- 200 g crème fraîche
- 200 g double cream
- 3 eggs
- 1/2 tsp salt
- 1 tsp vegetable stock powder
- 2 tsp oregano
- 200 g feta
- 16 leaves parsley
Instructions 0 / 10
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1
Prepare the vegetables.
Peel the sweet potatoes and cut into slices 3 mm thick. Wash the pepper, halve it, remove the stalk and seeds, and cut into strips. Clean the mushrooms and slice them.
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2
Preheat the oven.
Preheat the oven to 180°C.
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3
Grate the cheese.
Place the Gouda in the mixing bowl and chop for 4 sec / speed 8, then set aside.
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4
Ginger.
Peel the ginger and the garlic clove. Add both to the mixing bowl and chop for 4 sec / speed 8, then scrape down with the spatula.
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5
Butter.
Add the butter and steam for 3 min / Varoma / speed 1.
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6
Mushrooms.
Add the mushrooms and heat for 5 min / Varoma / reverse direction / speed 1.
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7
Sauce.
Add the crème fraîche, double cream, eggs, salt, vegetable stock powder and oregano and mix for 20 sec / reverse direction / speed 3.
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8
Layering.
Layer the sweet potatoes, pepper, sauce and crumbled feta alternately in a baking dish, finishing with a layer of sauce, then scatter the grated Gouda on top.
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9
Bake.
Bake the gratin on the middle shelf of the oven for 55 minutes.
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10
Serve.
Serve garnished with parsley.
Tip: Instead of Gouda, you can also use Parmesan. This makes the gratin a little more savoury.
Video
Nutrition per serving
Why the 3 mm slice is the key step
Sweet potatoes are not regular potatoes. Sweet potatoes need more heat and thinner slices. A waxy salad potato sliced at 5 mm will soften in 45 minutes. Sweet potatoes have more fibre, less starch and react more slowly to heat. At 6 mm and 180°C they are still firm in the centre after 55 minutes, the edges overcook and the crust on top burns. 3 mm is the threshold at which the sauce can soak through between the slices and everything finishes at the same time.
The Thermomix® handles two flavour steps in a row without us needing a second pan. First the ginger and garlic are chopped for 4 seconds at speed 8, then steamed together with the butter for 3 minutes at Varoma, speed 1. Straight after that, the mushrooms go in and cook for 5 minutes at Varoma in reverse direction. The mushrooms release water, some of it evaporates, and the rest flavours the sauce later on. In a frying pan that would mean spattering everywhere; in the mixing bowl it happens without any supervision.
The sauce made from crème fraîche, double cream and eggs binds itself. 200 g crème fraîche, 200 g double cream, three eggs, half a teaspoon of salt, one teaspoon of vegetable stock powder and two teaspoons of oregano go in for 20 seconds in reverse direction at speed 3. That is all. In the oven the eggs set, the cream reduces slightly, and the crème fraîche keeps everything creamy rather than rubbery. If you use flour here, you get a béchamel bake, not a gratin.
Layering and the cheese crust
The order of the layers in the baking dish matters. We start with a layer of sweet potatoes, then a third of the pepper and mushrooms, then some sauce, then crumbled feta. We repeat this twice, and finish with a layer of sweet potatoes and the remaining sauce. We scatter all the grated Gouda on top. The 50 g of Gouda are chopped for 4 seconds at speed 8 beforehand, which is enough for an even crust.
Important: the Gouda goes in right from the start because this recipe uses 180°C, not 220°C. At higher heat we would only add the cheese for the last 10 to 15 minutes. This way it stays golden rather than turning black, and the sweet potatoes underneath have time to cook through.
Where sweet potato gratin turns soggy or burns
Sweet potatoes sliced too thick
The most common problem with sweet potato gratin. Using a kitchen knife, every other slice ends up at 4 or 5 mm, and suddenly the centre of the gratin is still firm after 55 minutes. Our solution: We use a mandoline or a vegetable slicer set to 3 mm. If you do not have one, keep the knife at a shallow angle and cut deliberately thin. It is better to measure once than to spend an hour in the oven and end up disappointed.
Mushrooms added raw to the dish
If you lay the mushrooms uncooked between the layers, you end up with a watery sauce that pools at the bottom of the dish. Mushrooms are 92 per cent water and release all of it into the cream. Our solution: The 5 minutes at Varoma in reverse direction beforehand are essential. The water that evaporates in the mixing bowl at this stage would otherwise run through the entire gratin.
Feta in large cubes instead of crumbles
Large pieces of feta do not melt. They sit between the layers like white blocks, contributing neither salt nor creaminess, and pull the sauce apart. Our solution: We crumble the 200 g of feta roughly with a fork in a bowl before layering. Thumbnail-sized pieces are enough. They distribute evenly, add seasoning and become almost creamy in the sauce.
Baking dish too small
In a small, deep dish the sweet potato layers stack up too high, and the bottom stays raw while the cheese on top is already browning. Our solution: Use a shallow dish roughly 25 by 30 cm. The total height of the layers should not exceed 5 to 6 cm. That way the heat cooks through evenly and the sauce can seep between the slices.
Variations we actually make
With Parmesan instead of Gouda: We swap the 50 g of Gouda for 40 g of freshly grated Parmesan. More savoury, saltier, darker crust. In that case we usually halve the salt in the sauce.
With baby spinach between the layers: 100 g of young spinach, blanched and squeezed dry, goes between the second and third sweet potato layer. It adds colour and cuts through the sweetness of the tubers.
Spicier with chilli flakes: Half a teaspoon of chilli flakes in the sauce in the mixing bowl. It works well with sweet potatoes because the sweetness tempers the heat.
Without mushrooms: If you do not like mushrooms, replace them with 250 g of courgette cut into thin slices. Courgette needs no pre-cooking, but it releases a similar amount of water in the oven. In that case we skip the reverse-direction step and layer the courgette straight in.
What to serve alongside
As a vegetarian main, a green salad with a lemon and mustard dressing is all you need. If you are serving the gratin as a side dish, it pairs well with pan-fried salmon, chicken breast or a herbed steak. A simple potato soup with the Thermomix® as a starter makes the meal complete. Leftover gratin can be placed on a slice of pizza dough made in the Thermomix® the next day and grilled briefly until the cheese bubbles again.
3 days in the fridge, crisped up again in the oven
Stored in a covered dish, the gratin keeps well in the fridge for three days. To reheat, place it covered in the oven at 160°C for 20 minutes, then uncover for a further 5 minutes so the crust gets its bite back. In the microwave the sauce quickly turns grainy, which is not our preferred method.
Freezing works in principle, but the sweet potatoes lose their texture when thawed and turn soft. We only freeze the gratin in individual portions when we know we will be reheating it as a quick weekday meal. Thaw overnight in the fridge, then reheat as described above. Fresh from the oven is noticeably better.
You might also enjoy: Pasta Bake with the Thermomix®.
