1 1/2 pounds boneless, skinless chicken breasts, cut into bite-sized pieces
1 1/2 pounds broccoli, cut into bite-sized florets
2 (8-ounce) bags mixed baby bell peppers (or 2 large bell peppers), cut into 1-inch chunks
3 tablespoons olive oil
4 cloves garlic, smashed
1 tablespoon coarsely chopped fresh rosemary leaves, or 1 teaspoon dried Italian seasoning or Italian herb blend
1 teaspoon kosher salt
1/4 teaspoon freshly ground black pepper
Arrange a rack in the middle of the oven and heat to 500°F. Place a rimmed baking sheet in the oven while it is heating.
Place all the ingredients in a large bowl and toss to combine. Remove the hot baking sheet from the oven. Add the chicken and vegetable mixture and arrange into a single layer.
Return to the oven and turn the heat down to 475°F. Roast, stirring halfway through, until the chicken is cooked through and the vegetables are tender, 15 to 20 minutes total.