Preheat oven to 375°. Flatten chicken breasts to 1/2-in. thickness; place each on a piece of heavy-duty foil (about 12 in. square). Layer the mushrooms, carrots, onions and red pepper over chicken; sprinkle with pepper, thyme and, if desired, salt. , Fold foil around chicken and vegetables and seal tightly. Place on a baking sheet. Bake until chicken juices run clear, about 20 minutes. If desired, serve with lemon wedges.