Potato casserole with sprats

Wash the potatoes and boil in a uniform. Clean from skin and cut into slices. Finely chop the peeled onions and brown lightly over a medium heat. Peel the champignons, cut into plates and add to the onion. Fry the mushrooms, stirring, over high heat until the liquid evaporates and a ruddy crust appears. Place the mushrooms with the onions from the pan, expressing the oil. Take the sprats out of the oil, break into pieces of
1. 5-2 cm each. Put the potatoes in a form lubricated with oil, salt, a layer of fish on it, then mushrooms with onions. Lightly beat the egg with a fork with sour cream, salt, pepper, soda and pour over the tin. Bake in the oven at 180 ° C for 20-30 minutes. The dish can be served both hot and cold.