Preheat oven to 375 F. In 2-quart saucepan over medium heat, in hot butter, cook onion until tender, about 5 minutes. Stir in flour and salt until blended; cook 1 minute. Gradually stir in milk, cream and cook, stirring constantly, until mixture thickens slightly. Remove saucepan from hear; stir in 1/4 cup shredded cheese. In shallow 2-quart casserole, arrange half of the potato slices; pour half of the sauce on top; sprinkle with half of remaining cheese; repeat. Sprinkle with paprika. Bake, covered 45 minutes. Uncover and bake 15 minutes longer or until potatoes are tender.