Wash and peel the potatoes and chop into thin round slices.
Grease the casserole dish and layer the sliced potatoes across.
Cook in preheated oven for 25 minutes at 200C° (400F°).
While waiting, mix the crème fraîche (heavy cream), butter, garlic, salt and pepper in a bowl.
Lightly wipe the mushrooms with a damp tissue paper to make sure they are clean.
Once the 25 minutes of cooking is done, remove the casserole dish carefully from the oven and slide the mushrooms (with the caps facing up) in between the potatoes with a fork.
Add the creamy mixture over the top of the mushroom caps and across potatoes in the casserole dish.
Sprinkle the grated cheese on top.
Add a few leaves of parsley on top for decoration.
Continue cooking in the oven for 20 minutes at 200C° (400F°) until potatoes, mushrooms, and cheese are lightly browned.
Serve while warm.