Prepare the sauce. Melt the butter. Mix it with sour cream, salt, pepper and finely chopped garlic.
Salt and pepper the fish fillet. Grease a baking dish with vegetable oil. Place the fish pieces tightly next to each other. Place the onion rings on top of the fish. Pour the sauce over the fish so that it completely covers the pieces. Place the fish in the oven, preheated to 200 ℃, for 25-30 minutes.