Peel and core apples and cut into slices. Brush a shallow rimmed baking dish with butter. Mix crème fraîche with sweet whipped cream. Later, place apple slices in casserole dish in a flaky manner, drizzle with juice of one lemon. Sprinkle chopped walnuts on top of the first layer of apples and repeatedly put a layer of apples on top of it, repeatedly acidify with lemon and sprinkle the remaining walnuts on top. Baste everything with the crème fraîche-cream mixture, which is a little firm.
Top with butter flakes and put in the preheated oven (200 °C convection oven without broiler) for 20-25 min. The crème fraîche-cream mixture will thicken and coat all the apple slices, combining the whole into a delicious gratin.
Remove from the heat, cool slightly and serve with game, poultry or solo, lukewarm as a dessert.