Heat fat in a separate pan. Add onion and sauté it slowly for 10 minutes. Then add the flour; stir it for a few minutes but do not brown. Dilute with ¼cup water, whip till smooth, and mix in with sauerkraut. Simmer for an additional 10 minutes.
Remove head and liver from pike and cut remaining pike into serving pieces. With a wooden spoon, make space for each piece of pike in the sauerkraut. Place liver and head in the center (so you can find it easily). Pour bread crumbs over and dot with butter.
Bake it in a 375°F. oven for about 30 minutes, or until fish is done but not overdone. Do not stir. If you think the sauerkraut is sticking on the bottom of the pot, grasp handles of pot and make a fast twist-turn.
For traditional serving, put the liver in the mouth of the fish head, and put the head on top in the center of the platter. (This can be omitted for the squeamish.) Sour cream on top is optional.