Minced meat mixed with 1 egg, add salt (if not salted), pepper and spices to taste (I added herbs and spices to mix grill ), mix well. Home in refrigerator while preparing other products. Cut the onion in smaller washers and stew in fat (the butter and the oil) to transparency. Add the sliced ​​mushrooms, add salt and sauté until soft. Cut the tomatoes into slices (it is desirable to peel, I missed to do). Grate the cheese. In tray is put baking paper (or smeared with grease). The mixture of minced meat form 6 flat meatballs, they shall bear mushroom mixture on it put tomato slices and smeared with mayonnaise (to taste, I put 2 tsp each meatball). Top put the grated cheese. Bake in an oven preheated to 200 C, for 30-40 minutes, until the cheese is golden color.