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1. Prepare the caramelized onions:
Heat the oil and butter in a frying pan over medium heat.
Add the sliced onions and fry gently for 15 to 20 minutes until golden and slightly caramelized. Set aside.
2. Assemble the gratin:
Preheat your oven to 180°C (gas mark 6).
In a gratin dish, place a layer of potatoes. Season with salt, pepper and sprinkle with thyme.
Add a layer of caramelized onions, then a little grated cheese.
Repeat the layers until you have used up all the ingredients, finishing with a generous layer of cheese.
3. Add the liquids:
Mix the crème fraîche and stock, then pour this mixture over the gratin. Shake the dish lightly to distribute the liquid evenly.
4. Cook:
Cover the dish with aluminum foil and bake for 40 minutes.
Remove the aluminum foil and brown for another 15 to 20 minutes until the top is golden brown.
5. Serve:
Let stand for 5 minutes before serving so that the flavors blend perfectly.
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