Preheat oven to 350°.
Combine squash and water in a large microwave-safe bowl. Cover with plastic wrap; vent. Microwave at HIGH 6 minutes or until tender. Drain well.
Remove casing from sausage. Cook sausage, onion, and garlic in a large nonstick skillet over medium-high heat until browned, stirring to crumble. Drain.
Place the bread in a food processor, and pulse 10 times or until coarse crumbs form to measure 1 cup. Combine the squash, sausage mixture, 1/2 cup breadcrumbs, sour cream, cheese, salt, pepper, and soup. Spoon squash mixture in a 2-quart casserole coated with cooking spray.
Top with remaining 1/2 cup breadcrumbs. Spray breadcrumbs with cooking spray. Bake at 350° for 30 minutes.