Heat fat in the skillet and brown meat on both sides. Remove to a saucepan. Add 3 tablespoons water and onions. Cook on low heat for 2 hours. For the last half hour add mushrooms. Grind everything. Add seasoning, and mix well. To make dough: Cut the butter into the flour with a knife and rub in with fingertips. Add baking powder, egg yolks, egg and sour cream. Knead dough for few minutes. Roll out 2 rectangles 18 x 15 cm. For a line of stuffing 3 cm off one long side of each rectangle. Fold the dough over the stuffing, brush with egg white. Cut into 26 patties. Place on a buttered cookie sheet. Bake in a moderate 180ºC . oven for 35 minutes.