Put porcini mushrooms in a colander or sieve, set it in a bowl and defreeze on the coldest shelf of the refrigerator. Peel potatoes, cut into quarters, cover with boiling water with the addition of liquid from the defrosted mushrooms, salt and cook to absolute softness for 20-25 min. Drain the water, dry the potatoes, mash them adding sour cream. Cool down completely. Clean champignons, cut into thick plates. Lay 2 pieces of foil on a cooking surface. Distribute champignons and porcini mushrooms, season with salt and pepper, and top up with melted butter. Fold the edges of the foil to get high sides, but do not cover the mushrooms completely. Place on a baking sheet and put in the oven preheated to 200 °C, bake for about 20 minutes until the liquid evaporates. Peel onion, chop up and fry in oil for 10 minutes. Chop the baked mushrooms in a blender not too finely, add salt and pepper. Beat up eggs, mix with the mashed potatoes. Add flour gradually to get an elastic and not very sticky mass. Oil the center of a 30x30 cm film and put about 2 tbsp. potato mass. Make a deepening in it, put a little mushroom filling and close with a potato. Shape cutlets with the help of a film, tightening it like a sack. Roll the cutlets in breadcrumbs, fry in oil on both sides to golden brown and then place to a foil-covered baking sheet. Cook in the oven for about 15 minutes. Serve with sour cream.