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Potato Gratin with Mushrooms in the Oven
Instructions
Chop the onion into large pieces, 7–8 mm. Coarsely cut onion gives the gratin a pleasant texture, while finely chopped onion dissolves into the sauce without a trace. For French cooking I always cut the onion coarsely.
Saute the onion in vegetable oil for 5–7 minutes over medium heat until translucent and lightly golden – onion cooked like this gives the sauce a caramel sweetness.
Slice the champignons thinly, 2–3 mm. It is best to use small champignons – they are more delicate in flavour. Before slicing, be sure to rinse the mushrooms and pat them dry with paper towel.
Add the chopped mushrooms to the fried onion. Fry for 5–7 minutes until lightly golden. It is important to keep the mushrooms juicy, as they will bake further in the oven. Overcooked mushrooms lose their structure and make the sauce watery.
Slice the peeled potatoes into rounds 1–2 mm thick. The thinner the potatoes and the more layers, the better they bake and the softer the texture. I use a special mandoline slicer for evenly thin slices – it is hard to get them uniform with a knife.
Lay the first thin layer of potatoes in a greased dish. Sprinkle with the pepper mix, sea salt and thyme. Spread a little of the fried mushrooms on top. Salt every layer – it is important that the salt is distributed evenly throughout the gratin.
Drizzle the potato layer with sour cream (you can dilute it 50% with cream for extra tenderness) and sprinkle generously with freshly chopped dill. The sour-cream sauce soaks into the potatoes and makes them melt in the mouth.
Repeat all the layers until the dish is full – you should get 4–5 layers. Sprinkle the top layer with vegetable oil to form a crust and put it in the oven for 45 minutes at 180 °C.
Take the finished gratin out and let it rest for 5 minutes under foil, so the layers firm up and do not fall apart when serving. Transfer the casserole to a serving plate and finish with a fresh basil leaf.
Tips
- 1
Slice the potatoes as thinly as possible, 1–2 mm – that way they bake better and the layers turn out more tender, melting in the mouth.
- 2
Do not overcook the mushrooms – they should stay juicy before baking, otherwise they turn into dry, rubbery bits after the oven.
- 3
You can replace the sour cream with bechamel sauce or 33% cream – the flavour will be different but no less interesting. A potato casserole is made on a similar principle.
- 4
Experiment with herbs – rosemary, thyme and oregano are all great for gratin and give it a restaurant character.
FAQ
Which mushrooms are best to use for gratin? +
Small champignons are ideal – tender, mild and easy to find. You can add oyster mushrooms or porcini for a richer, more woodland flavour. Wild mushrooms (orange-cap boletus, birch boletus, chanterelles) give the brightest aroma, but boil them first for 10–15 minutes in salted water. Dried porcini (5–10 g) give a concentrated aroma – soak them for an hour and use them together with the soaking water. Oyster mushrooms are best combined with champignons – they give a pleasant contrast in texture.
Can the gratin be made without sour cream? +
Yes, replace the sour cream with 33% heavy cream (for a creamier taste), classic bechamel sauce (milk + flour + butter), Greek yoghurt (a lighter option) or processed cheese (a cheesier taste). Without a liquid sauce the gratin will turn out dry – it is its basic component. The most authentic French version is thick cream with grated Gruyere cheese on top. For a vegan version, use coconut milk with nutritional yeast – it gives a similar "cheesy" effect.
At what temperature should the gratin be baked? +
180 °C for about 45 minutes is the classic combination for even baking and a golden crust. Check for doneness with a toothpick – the potato should be soft when pierced. At 200 °C the time drops to 35 minutes, but the crust may burn before the middle is cooked through. At 160 °C it needs an hour and the gratin comes out more tender. For a browned top, switch on the top heat or grill mode for the last 5–10 minutes.
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