Balsamic Chicken with Leeks and Peppers

1 medium onion, cut into wedges
2 medium leeks, trimmed and sliced
1 medium sweet red pepper(s), sliced in half, stem and seeds removed
1 medium yellow peppers, sliced in half, stem and seeds removed
2 tsp olive oil
2 rosemary sprig
1/8 tsp table salt, or to taste
1/8 tsp black pepper, or to taste
1 1/4 pound uncooked boneless, skinless chicken breasts, four 6 oz pieces
1 Tbsp balsamic vinegar

Preheat oven to 375°F. Put onion, leeks and peppers into a roasting dish. Add oil and toss to coat. Add rosemary; season to taste with salt and pepper. Place chicken on top of vegetables. Sprinkle with vinegar, season to taste and transfer to oven.

Roast until chicken is browned and cooked through, and vegetables are tender, about 25 to 30 minutes. Remove from oven and slice peppers. Serve.