In a saucepan, sauté onion and garlic in butter; cook until tender. Whisk in flour, salt and pepper. Gradually whisk in the milk.
Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in horseradish and sweet potatoes.
Transfer half of the sweet potato mixture to a greased 9-inch square baking dish coated with cooking spray.
Combine cheese and bread crumbs; sprinkle half the crumb mixture over the sweet potatoes.
Cover the crumb mixture with the rest of the potatoes.
Sprinkle the remaining crumbs mixture over the potatoes.
Bake for 30 minutes at 350°or until golden.