Bone the herring and cut into small pieces. Slice some cooked potatoes; then butter a baking-dish; sprinkle with flour. Put a layer of potatoes, some chopped onion and herring and bits of butter until dish is full; sprinkle with pepper. Make the top layer of potatoes and bits of butter. Moisten with 3 tablespoonfuls of sour cream. Bake in a moderate oven until brown. Serve hot.