Rinse the lettuce, and set aside 8 leaves.
In a large skillet over medium heat, melt the butter.
Add onion, pepper, and garlic cloves, cooking until soft and browned, about 7-8 minutes.
Add diced chicken and water chestnuts, cooking until heated through, about 3 minutes.
Add sesame oil, soy sauce, and P.F. Chang's® Home Menu Mongolian Sauce, cooking until heated through, about 1-2 minutes.
Spoon chicken mixture into 8 lettuce leaves, and serve immediately.