In a small bowl, toss the tofu with soy sauce.
For a seasoning sauce, stir together mushroom sauce with the white pepper and rice vinegar and chili sauce if desired.
Heat a wok or large skillet over medium heat, add the sesame oil and then fry the tofu. Shake and turn to richly brown the tofu as best you can. This takes 3 or 4 minutes. Push the tofu to the side, increase the heat to high, then add the mushrooms. Cook, stirring for about 5 minutes until browned. Add green onions and stir to combine mushrooms and tofu well. Pour in the seasoning sauce and keep working the wok, till most of the liquid has disappeared, about 45 seconds.
Remove from the heat, taste and make any flavor adjustment with a bit of salt or extra white pepper. Transfer to a bowl or plate. To serve, put some of the stir-fried mixture in a lettuce cup, bundle it up and add sauce if desired.
I served mine with this Peanut Sauce and a Soy, vinegar chili pepper sauce.