Peel the onion, cut into thin rings. Heat some margarine in a pan. Let the onions become translucent in it, then pour them into a baking dish.
Wash the pollack fillet and sea crabs and pat dry with kitchen paper. Put the saithe with the onions in the baking dish, season with salt and spread the sea crabs on top. Mix the sour cream and the broth powder, pour over the crabs. Now bake the fish casserole in the preheated oven at 180 degrees (gas level 2, convection 160 degrees) for about 30 minutes.
Serve immediately afterwards. Served with firm cooked rice.