Clean the onions and blend them in a blender.
In a bowl, put the mince, onions, egg, savory, black pepper and salt to taste, knead well.
Chop the mushrooms finely, braise in the melted butter to a light red, leave them to cool. Grate the potatoes and salt them lightly to taste.
Divide up the mince into 4 equal parts, shape them into round and slightly indented schnitzels (like a bowl), put them in an oiled tray.
Among them, evenly distribute the braised mushrooms, then potatoes, the mayonnaise and finally grate the cheese evenly on top.
Put the tray in a preheated 356°F (180 °C) oven and bake about 30 min. Serve with a fresh salad.