Potatoes au Gratin with Sweet Vidalia Onions

Combine potatoes and onion in a greased one-quart casserole and set aside. In medium saucepan, melt the butter over medium heat, then stir in flour until smooth. Gradually add milk, stirring constantly until mixture thickens and begins to bubble. Add salt, pepper, and garlic, then gradually blend in cheeses, stirring until the mixture is uniform and smooth. Pour cheese sauce over potato mixture and stir once or twice to mix. Sprinkle paprika lightly over potato-cheese blend. Bake at 350 degrees F for 40 minutes or until bubbly and beginning to brown on top.