Cook the milk with processed cheese on a low setting, stirring constantly, until the cheese has completely dissolved. Add the peas and mushrooms, season with mustard and remove from the fire.
Place the steaks in a baking dish and spread the sauce over them. Sprinkle the Emmentaler cheese on top and spread the sliced mozzarella over it. You can sprinkle parsley or other herbs on top as you like.
Baked in the oven at 200 ° C for about half an hour (depending on the size of the steaks).