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.