Cut acorn squash into halves and remove seeds.
Oven-steam squash, cut side down, in covered baking pan with 1/2 inch of boiling water in preheated hot oven (400Â°F.) until tender.
Meanwhile, mash sausage with fork and fry until cooked but not brown.
Drain off fat.
Add onion, bread crumbs, and salt and pepper to sausage.
Fill centers of squash halves with mixture.
Reduce oven heat to moderate (375Â°F.) and bake uncovered for about 30 minutes longer.