Preheat oven to 400°.
Place a nonstick skillet coated with cooking spray over medium heat until hot. Add onion; sauté 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. Bake at 400° for 25 minutes or until golden. Let stand 5 minutes.