Preheat the oven to 400 degrees F.
Cut the larger Brussels sprouts into quarters; halve the smaller ones.
Place the Brussels sprouts in a large baking dish or on a baking sheet. Add the garlic, sprinkle with salt and pepper, and pour over the olive oil. Toss to coat. Transfer to the oven and roast for 35 minutes. Crank up the oven to 450 degrees F and roast until tender and the outer leaves are browned and crispy, another 10 minutes. Transfer to a serving dish. Serve immediately.