Preparation Preheat oven to 350°F. Bake potatoes 30 minutes. Using fork, pierce potatoes in several places. Bake until tender, about 1 hour longer. Maintain oven temperature. Meanwhile, microwave milk and rosemary in microwave-safe cup on high heat 45 seconds; let steep 30 minutes. Slicing lengthwise, cut top quarter off each potato. Scoop potato into large bowl, leaving about 1/4 inch of potato in skin. Scrapepotato out of tops; discard tops. Coarsely mash potato. Add 4 tablespoons butter; mash. Mash in milk mixture, then Roquefort cheese (mixture will be slightly stiff). Season with coarse salt and pepper. Divide mashed potatoes among potato skins; transfer to rimmed baking sheet. Sprinkle Gruyère cheese over potatoes, pressing slightly to adhere. Dot with butter. DO AHEAD: Can be made 4 hours ahead. Let stand at room temperature. Bake potatoes until hot and tops begin to brown, about 30 minutes.
Preparation Preheat oven to 350°F. Bake potatoes 30 minutes. Using fork, pierce potatoes in several places. Bake until tender, about 1 hour longer. Maintain oven temperature. Meanwhile, microwave milk and rosemary in microwave-safe cup on high heat 45 seconds; let steep 30 minutes. Slicing lengthwise, cut top quarter off each potato. Scoop potato into large bowl, leaving about 1/4 inch of potato in skin. Scrapepotato out of tops; discard tops. Coarsely mash potato. Add 4 tablespoons butter; mash. Mash in milk mixture, then Roquefort cheese (mixture will be slightly stiff). Season with coarse salt and pepper. Divide mashed potatoes among potato skins; transfer to rimmed baking sheet. Sprinkle Gruyère cheese over potatoes, pressing slightly to adhere. Dot with butter. DO AHEAD: Can be made 4 hours ahead. Let stand at room temperature. Bake potatoes until hot and tops begin to brown, about 30 minutes.