In a medium bowl combine soy sauce, fish sauce, brown sugar, rice wine vinegar, and sesame oil. Set aside.
Heat a wok or large sauté pan over medium-high heat, add the vegetable oil and heat through. Add the onion and garlic and sauté until the onions are soft and translucent, about 4 minutes.
Add the whites of the green onion, water chestnuts and mushrooms. Continue to sauté, stirring occasionally until tender, about 4 minutes.
Add the chicken to the skillet and pour the sauce over the mixture. Stir to combine and continue to cook, stirring occasionally, until the chicken has cooked through and the sauce has thickened (it should be the consistency of syrup), about 10 minutes. Remove from heat and stir in the remaining green onion.
To serve: scoop the hot chicken mixture into lettuce cups with jasmine rice. Garnish with Fresno peppers and micro cilantro (optional).