Cook the onions in the butter until golden, stirring almost constantly.
Scrub and trim the squash and cut them into cubes. Cook squash in 1 inch of boiling water containing the salt for 5 to 10 minutes, or until tender. Put the squash through a food mill or ricer.
Combine the puréed squash, onions and sour cream and season the mixture with additional salt and pepper to taste. Spoon it into a casserole and top with crumbs. Dribble the butter over it.
Bake for 10 to 15 minutes, or until the casserole starts to bubble.
Place it briefly under the broiler until crumbs are brown.