1Heat gas or charcoal grill. In small bowl, mix mustard, honey, oregano and red pepper. Brush on both sides of chicken.
2Place chicken on grill over medium heat. Cover grill; cook 12 to 15 minutes, brushing frequently with mustard mixture and turning occasionally, until juice of chicken is clear when center of thickest part is cut (at least 165°F). Discard any remaining mustard mixture. Serve chicken in buns with tomato and lettuce.