Heat oven to 425°F. On a large rimmed baking sheet toss breussel sprouts, onion, 2 Tbsp. oil, 1/2 tsp. salt, 1/4 tsp. pepper. Roast until golden brown, 20 to 25 minutes.

Meanwhile, heat the remaining Tbsp. oil in a large skilled over medium heat. Season the chicken with 1/2 tsp. each salt and pepper and cook, skin-side down, until the chicken is crisp, 7 to 8 minutes.

Turn the chicken, add the garlic and rosemary to the pan and cool until the garlic is golden brown and the chicken is cooked through, 5 to 6minutes more. Serve with the vegetables.