Preheat oven to 400 F. In a large saucepan, add sweet potatoes, onion, water, rosemary and salt. Bring to a boil. Reduce heat and simmer for 15 minutes or until potatoes are tender. Drain potatoes, reserving the cooking liquid for the sauce. Transfer potatoes and onions into baking pan. In medium saucepan, melt butter, blend in flour, add milk and potato liquid; stir constantly until thickened. Pour sauce over the potatoes. Sprinkle with Parmesan cheese and paprika. Bake for 10-25 minutes, until cheese is melted. Makes 4-6 servings.