Dice the chicken into small, uniform pieces (about 1/4 inch). Finely chop the shiitake mushrooms, drain and dice the water chestnuts, finely mince the yellow onion, garlic, and ginger. Separate and wash the bibb lettuce leaves, set upright on a towel to drain.

Cook aromatics & mushrooms

Heat a large skillet or wok over medium-high heat. Add 1 tablespoon vegetable oil. When shimmering, add the minced onion and the white parts of the sliced green onions. Sauté 2–3 minutes until softened. Add the mushrooms and cook 3–4 minutes until they release moisture and begin to brown. Stir in garlic and ginger and cook 30–45 seconds until fragrant.

Cook chicken

Push the vegetables to the side of the pan, add the remaining 1 tablespoon oil to the center and add the diced chicken in an even layer. Let it sear 1–2 minutes without moving, then stir to break up and cook through about 3–4 minutes until no longer pink. Mix the chicken with the vegetables and mushrooms.

Add water chestnuts & green onions

Stir in the diced water chestnuts and the white-green mixture; cook 1 minute to warm through while keeping the chestnuts crisp.

Sauce & Seasoning

While the chicken cooks, whisk together soy sauce, oyster sauce, hoisin, rice wine, sesame oil, chicken broth (or water), and brown sugar in a small bowl until combined. If you prefer a thicker sauce, dissolve the cornstarch in a tablespoon of cold water and set aside.

Pour the sauce into the skillet with the cooked chicken and vegetables. Stir to combine and simmer 1–2 minutes. If using slurry, add it now and stir; simmer another 30–60 seconds until the sauce thickens and coats the filling. Taste and adjust: add a pinch more sugar if too salty or a teaspoon water if too thick.

To Serve

Transfer the filling to a warm bowl. Arrange bibb lettuce leaves on a platter. Spoon about 2–3 tablespoons of the hot chicken mixture into each leaf (adjust to preference).

Garnish with sliced green onion tops and toasted sesame seeds. Serve immediately with sriracha or chili sauce on the side for those who want heat.