Place a nonstick skillet coated with cooking spray over medium heat until hot.
Add onion; saut 5 minutes or until tender. Stir in flour. Gradually add milk; cook 10 minutes or until slightly thick, stirring constantly with a whisk.
Remove from heat; add cheese, sage, garlic powder, salt, and pepper, stirring until cheese melts. Stir in rutabagas.
Spoon into a 1 1/2-quart shallow baking dish coated with cooking spray; sprinkle with breadcrumbs.