Skin and bone the fish and cut it in 2 inch lengths. Make fish stock with the bones, sliced carrots, 1 onion, salt and pepper. Boil the sauerkraut till tender, and drain thoroughly. Make a sauce with the butter and flour, mixed and stirred till a golden colour, 1 cooked finely-chopped onion, the ham, ½ pint of fish stock, the gill of sour cream. Mix all well, and simmer till slightly reduced. Put a layer of sauerkraut in the bottom of a fireproof dish, put pieces of fish over it, a little sauce, another layer of sauerkraut, then more fish and sauce. Sprinkle with grated cheese, and put in the oven for 30 minutes or longer, according to what fish is used.