In a large saucepan, melt butter over medium heat. Stir in flour until smooth. Gradually add the cream. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat. Stir in the rosemary, garlic, salt, pepper and potatoes.
Transfer to a greased 13-in. x 9-in. x 2-in. baking dish. Sprinkle with Parmesan cheese. Cover and bake at 350 degrees F for 45 minutes. Uncover; bake 10-15 minutes longer or until potatoes are tender.