Prep the greens. Gently separate the lettuce leaves, rinse, and pat dry. Choose sturdy leaves that form cups. Set aside.
Mix the sauce. In a small bowl, whisk tamari, vinegar, sesame oil, chili-garlic sauce, fish sauce, and sweetener.
Taste and adjust salt and pepper.
Sauté aromatics. Heat oil in a large skillet over medium-high. Add onion, garlic, ginger, and the white parts of the green onions. Cook 2–3 minutes until fragrant.
Brown the chicken. Add ground chicken.
Break it up with a spatula and cook 5–7 minutes until no longer pink. If using mushrooms or bell pepper, stir them in now and cook another 2–3 minutes.
Add the sauce. Pour the sauce over the chicken mixture. Stir and cook 1–2 minutes until slightly reduced and glossy.
Adjust seasoning and heat level.
Assemble. Spoon the chicken mixture into lettuce cups. Top with green onion tops, cilantro, sesame seeds, and a squeeze of lime. Serve immediately.