Cook squash and onion in a small amount of boiling water 10 to 12 minutes or until vegetables are tender. Drain; set aside. Melt margarine in a medium size, heavy saucepan over medium heat. Add flour; cook, stirring constantly, 1 minute.
Gradually add milk; cook, stirring constantly, until mixture is thickened and bubbly. Remove from heat; add cheese, salt, and pepper, stirring until cheese melts. Add squash mixture; stir well. Spoon squash mixture into a shallow 1 ½ quart baking dish coated with cooking spray. Sprinkle squash mixture evenly with breadcrumbs. Bake at 350º for 20-25 minutes or until mixture is thoroughly heated.