Cook rice and set aside.
Combine the soy sauce, hoisin sauce, rice vinegar, salt, and sugar in a small bowl and mix together until the sugar dissolves.
Heat 1 tablespoon of the oil in a wok or large skillet over high heat. Stir-fry the garlic and ginger for 10 seconds. Add the onion, pepper, mushrooms, and water chestnuts and stir-fry for 3 minutes.
Add cooked rice into wok. Lower the heat, and stir in the sauce mixture. Stir for 1-2 minutes, or until the sauce is heated.
Spoon the filling in equal amounts into the lettuce cups. Top each lettuce cup with cilantro, parsley, and chives. Serve warm.