Cut the fillet into bites and braise them in a little oil. Once reddened, remove from the pan, salt lightly and season with black pepper.
Saute the mushrooms until softened, then salt them as well. Arrange the roundly sliced potatoes in a tray, arrange the braised meat on top.
Pour on the mushrooms, chop the processed cheese and butter and arrange them atop the mushrooms. Pour in 1 cup water and cover the tray tightly with aluminum foil.
Place in a preheated 392°F (200 °C) oven and bake 40 min. Remove the foil and bake another 10 min. Sprinkle grated cheese on top and leave it to crisp slightly.