Remove the tofu from its packaging and drain. Wrap tofu in a kitchen towel and find a heavy object to place on top of tofu, leave it there for 10-20 minutes, until tofu feels more tender.
Make the sauce: In a medium bowl, combine the water, soy sauce, sesame oil, garlic, ginger, sriracha, brown sugar, rice wine vinegar, and scallions, and stir. Set ½ cup (120 ml) of the sauce aside.
Crumble the tofu into the marinade, cover, and refrigerate for 1 hour.
In a medium wok or skillet over medium heat, add the sesame oil and onion, and cook 3 minutes, until onions are translucent. Add the mushrooms and garlic and cook for about 5 minutes, or until the mushrooms are tender.
Add the marinated tofu to the pan and cook until the majority of marinade cooks off. Add the water chestnuts and scallions. Pour in the reserved sauce. Toss to coat and cook until just heated through. Transfer the tofu mixture to a medium bowl.
Heat the canola oil in the same pan over high heat until a drop of water sizzles when flicked onto the oil. Fry the rice noodles for about 30 seconds, until puffed up. Transfer the fried noodles to paper towels to drain.
Slice the core off the head of lettuce and separate into individual leaves.
Serve with leaves of lettuce, rice noodles, and drizzle remaining sauce over top.