You can never have too many easy sheet-pan suppers in rotation.
On rimmed baking sheet, toss onion, garlic, rosemary, and oil; season. Season chicken on both sides with salt, pepper, and garlic powder. Arrange chicken, skin-side up, on top of onion mixture. Bake at 450° until chicken skin is golden, about 20 minutes. Add peppers to baking sheet. Drizzle with brine. Broil until chicken skin is deeply browned in spots, 2 to 3 minutes more. Sprinkle with parsley.