Boil the potatoes and prepare thick paste with milk (cream) and 50 g butter. Season to taste with salt, pepper and nutmeg. In a pan, heat about 30 g butter and fry the onion, add the minced meat. Fry, stirring 6-7 minutes. Season to taste with salt and pepper. Brush with butter walls of the vessel, which will be baked dish, pour half of the puree, the meat on top and the second half of the puree. Sprinkle top with grated feta cheese. Bake about 15 minutes in a preheated 200 ° C oven. To the meat can add mushrooms or chopped bacon, finely chopped fresh herbs to taste, etc.