Place potatoes in a greased 13-in. x 9-in. baking dish. Bake, uncovered, at 400degrees for 40 minutes., Meanwhile, in a small saucepan, melt butter. Stir in flour until smooth; gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in the cheese, rosemary, garlic, salt and pepper., Pour over potatoes and stir to coat. Bake 8-12 minutes longer or until sauce is bubbly and potatoes are tender.