Preheat oven to 400°. Cut each beet into 6 wedges; place in a shallow dish. Add olive oil and salt; toss gently to coat., Place a piece of heavy-duty foil about 12 in. long in a 15x10x1-in. baking pan. Arrange beets on foil; top with rosemary. Fold foil around beets and seal tightly. , Bake until tender, about 1 hour. Open foil carefully to allow steam to escape. Discard rosemary sprigs.