1. Prepare grill.
2. Cut baguette in half lengthwise. Hollow out top half of baguette, leaving a 1/2-inch border; reserve torn bread for another use. Set aside.
3. Place chicken breast halves between 2 large sheets of heavy-duty plastic wrap; pound to 1/2-inch thickness using a meat mallet or small heavy skillet. Sprinkle chicken with salt and pepper.
4. Flatten bell pepper quarters with hands. Coat chicken breasts and bell pepper quarters with cooking spray; place on grill rack. Cover and grill 8 minutes or until chicken is done and bell peppers are tender, turning once. Place cut sides of baguette halves on grill during last 2 minutes of grilling to lightly toast. Cut each chicken breast in half crosswise.
5. Spread both cut halves of baguette evenly with pesto. Top bottom half with lettuce leaves, chicken breast halves, bell pepper quarters, and cheese slices. Place top half of baguette on top of cheese. Cut crosswise into 4 equal portions.