Preheat oven to 375°F Place beets in roasting pan. Add 4 rosemary sprigs and enough water to barely cover beets. Cover pan tightly with foil. Roast beets until tender, about 50 minutes. Transfer beets to work surface. Peel while still warm; place on rimmed baking sheet. (Can be made 1 day ahead. Cover; chill.).
Preheat oven to 350°F Melt butter with oil in small saucepan. Pour over beets on sheet; toss to coat. Sprinkle with salt and pepper. Bake until heated through, stirring occasionally, about 20 minutes. Transfer to bowl. Garnish with additional rosemary sprigs and serve.