Preheat oven to 450 degrees.
Bring a medium saucepan of salted water to a boil over high heat. Add Brussels sprouts and cook for 2 minutes. Drain and transfer to a bowl lined with a clean kitchen towel; pat dry to remove any excess moisture. Remove towel and season Brussels sprouts with salt and pepper. Drizzle with olive oil and toss to coat. Transfer to a rimmed baking sheet, placing cut sides down, along with rosemary and garlic. Roast until browned and tender, about 20 minutes.