Finely chop the onion. Bulgarian pepper cut into cubes. Fry minced meat until half cooked. Add onion and pepper and fry for another 5 minutes. Add tomato paste, cream and mustard. Salt, pepper to taste. Add the mushroom slices and cook for 5 minutes.
Put a layer of mashed potatoes in a greased form. Then minced meat filling and again a layer of potatoes.
Beat the egg with sour cream and pour over the casserole mixture. Put in an oven preheated to 180 degrees for 30 minutes.