1. Heat an outdoor grill or skillet to medium-hot. Grill chicken breast, turning once, until golden brown and no longer pink in the center, about 15 minutes.
2. Slice the French roll in half horizontally, and spread yellow mustard on both sides.
3. Lay the grilled chicken on the bottom half of the roll, followed by the tomato slices and lettuce.
4. Reduce heat to low, and place the roll on the grill or skillet. Heat, turning once and flattening the sandwich with a spatula, for two minutes or until roll is lightly browned.
5. Cut in half and serve warm.

Makes 1 sandwich.