Prep the lettuce. Gently separate the leaves, rinse, and pat dry. Keep them chilled so they stay crisp.
Whisk the sauce. In a small bowl, mix soy sauce, rice vinegar, sweetener, hoisin (or substitute), sriracha, sesame oil, and water.
Taste and adjust sweetness and heat.
Sauté aromatics. Heat oil in a large skillet over medium-high. Add onion and mushrooms; cook 3–4 minutes until softened. Stir in garlic and ginger for 30 seconds, just until fragrant.
Brown the chicken. Add ground chicken, breaking it into small crumbles.
Cook until no longer pink and lightly browned, about 5–7 minutes. Don’t overcrowd—let some bits crisp for better texture.
Add crunch. Stir in water chestnuts and half the green onions. Cook 1 minute.
Sauce it up. Pour in the sauce and toss to coat. Simmer 1–2 minutes until slightly thickened and glossy.
If it seems dry, add a splash of water; if too thin, let it cook another minute.
Taste and tweak. Add more soy for salt, more sweetener for balance, or more sriracha for heat. Finish with a sprinkle of sesame seeds and remaining green onions.
Assemble and serve. Spoon the warm filling into lettuce cups. Garnish with cilantro and a squeeze of lime if you like.
Serve immediately while crisp and hot.