In a large cast-iron or other heavy skillet, cook and crumble chicken over medium-high heat 3 minutes. Stir in celery and pepper; cook and stir until chicken is no longer pink and celery is crisp-tender, 2-3 minutes., Stir in ketchup, hot sauce, brown sugar, vinegar and mustard; bring to a boil. Reduce heat; simmer, covered, until flavors are blended, about 5 minutes, stirring occasionally. Serve in lettuce leaves. If desired, top with cheese. Freeze option: Freeze cooled chicken mixture in freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally; add a little broth or water if necessary.