4largefresh rosemary sprigs,plus additional sprigs for garnish
Preheat oven to 375°F.
Leave beets unpeeled. Rinse the beets and trim, leaving 1-inch of the top greens attached.
Place the beets in a roasting pan. Add 4 rosemary sprigs and enough water to barely cover the beets. Cover the pan tightly with foil. Roast the beets until tender, about 50 minutes. Transfer the beets to a work surface. Peel while still warm; place on a rimmed baking sheet. (Can be made 1 day ahead. Cover; chill.)
When you are ready to prepare the beets for eating, make sure the oven is preheated to 350°F.
Melt the butter with the oil in a small saucepan. Pour over the beets on the baking sheet; toss to coat. Sprinkle with salt and pepper. Bake until heated through, stirring occasionally, about 20 minutes. Transfer to a bowl. Garnish with additional rosemary sprigs and serve.