Whisk together the sauce ingredients and set aside: hoisin, soy sauce, vinegar, ginger, chili and water.
Heat 1 tablespoon of the oil in a large skillet over medium heat. Add the tofu and cook until browned, about 12 minutes, stirring occasionally. The tofu should be dry, chewy and slightly crispy. Remove from pan and set aside.
Heat the remaining tablespoon of oil and add the mushrooms, onion and garlic. Cook, stirring often, until mushrooms are very browned and onions are soft.
Add the cooked tofu to the mushroom skillet along with the water chestnuts, and sauce. Cook another 2-3 minutes until the sauce is slightly thickened.
Remove from heat and top with chopped peanuts and scallions. Spoon into the lettuce cups and enjoy immediately.
Notes
To press tofu, wrap in paper towels and place a heavy object on top- or, use a tofu press.