Preheat the oven to 400 degrees F (200 degrees C).
Heat oil in a very large oven-safe skillet over high heat until shimmering. Arrange Brussels sprouts cut sides down in the skillet. Cook until cut sides are browned, 2 to 3 minutes. Stir in grapes and garlic. Top with rosemary sprigs.
Roast in the preheated oven until sprouts are tender, about 15 minutes.
Season with salt and black pepper. Squeeze lemon wedges over sprouts. If desired, remove rosemary before serving. Garnish with additional pepper.