Chop the onions and fry in a little butter to a golden brown. Mix thoroughly with the kascha, chopped hard-boiled eggs, herbs, and season with salt and pepper. Cook for a few minutes longer, and stuff the fish with the mixture. Put in a fireproof dish or in a baking tin, pour melted butter over the fish, add a little water in the dish or tin, sprinkle the fish with breadcrumbs and bake till tender. Before serving stir the sour cream into the butter in which the fish has cooked and pour this sauce over the fish when it is dished up.