Wash, dry and separate leaves of lettuce. Set aside.
In a medium skillet, over medium heat, warm up 1 tablespoon sesame oil. Add the chicken and the onion and cook until cooked thru, about 5 - 7 minutes. The onion will become translucent. Break up any chunks of chicken as it browns.
In a measuring cup, whisk together garlic, soy sauce, hoisin sauce, fresh ginger, rice vinegar and sriracha sauce. Pour into the skillet with the chicken. Add the water chestnuts. Stir until the chicken, onions and water chestnuts are coated and the sauce starts to thicken, about 5 minutes.
Remove from heat. Stir in 2 teaspoons of sesame oil. Salt to taste.
In a small bowl, mix the Ponzu with the Thai Chili Sauce.
Spoon about 2 tablespoons of the chicken mixture into a lettuce leaf, and top with sauce. Fold the lettuce over like a taco to eat.