1. Preheat oven to 450°F. Place peppers into a 13x9-inch baking dish. Sprinkle with 1/2 teaspoon fresh rosemary, salt, and black pepper to taste. Roast 20 minutes, stirring once. Meanwhile, sprinkle the 4 chicken breast halves with salt, pepper, and 1 teaspoon fresh rosemary.
2. Remove the peppers from the oven, stir, and tuck the chicken and garlic between the pepper pieces. Reduce oven temperature to 400°F. Roast, turning the chicken and rearranging the peppers and garlic once or twice, until chicken is cooked through but still juicy, and peppers are well done, 35 to 40 minutes.