Preheat oven to 375°.
Pierce potatoes with a fork, and bake at 375° for 1 hour or until tender. Cool potatoes slightly.
Melt butter in a large nonstick skillet over medium heat. Add onion and garlic; cook 20 minutes or until browned. Stir in chopped fresh rosemary and black pepper.
Split potatoes lengthwise, cutting to, but not through, other side. Spoon about 1/3 cup onion mixture into each potato. Sprinkle each serving with 2 tablespoons shredded Gruyére cheese. Broil 3 minutes or until cheese is lightly browned.