Combine mayonnaise and mustard in a small bowl; stir well.
Cut bread in half horizontally. Spread mayonnaise mixture evenly over cut sides of bread. Place bread halves, cut sides up, on a baking sheet. Place chicken on bottom half of bread; top with bell pepper and cheese slices, overlapping cheese slices as necessary.
Broil sandwich 2 minutes or until cheese melts. Place basil leaves over melted cheese; replace top half of loaf. Cut sandwich into 6 equal portions.