Chicken and Pesto Stacks

Preheat broiler. In small bowl, mix pesto with water. Spray 15 1/2" by 10 1/2" jelly-roll pan with nonstick cooking spray. Arrange onion slices on pan; brush onion slices with 2 tablespoons pesto mixture. Place pan in oven 6 inches away from heat source; broil onion 5 minutes. Arrange peppers on pan with onion; broil 12 to 15 minutes longer or until vegetables are tender, turning vegetables over halfway through cooking. Meanwhile, with meat mallet, pound chicken (placed between 2 sheets plastic wrap) to an even 1/2-inch thickness; season with 1/4 teaspoon each salt and freshly ground black pepper. In 12-inch skillet, heat 2 teaspoons oil on medium until hot. Add chicken and cook 6 to 8 minutes or until browned on both sides, turning over once. Top chicken in pan with mozzarella slices; cover and cook 2 to 3 minutes or until cheese melts and chicken is no longer pink throughout. To serve, on each of 4 dinner plates, place 1 slice onion, 2 slices pepper, 1 slice tomato, and 1 piece chicken and mozzarella; drizzle with pesto mixture.