Preheat oven to 400°.
Cut each beet into 6 wedges; place in a shallow dish.
Add olive oil and salt; toss gently to coat.
Cut a piece of heavy-duty foil about 12 in. long; place 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.
