If you have time, press the tofu for 15–30 minutes to remove excess moisture for the best texture. If not, simply pat it dry well with paper towels. To press the tofu, wrap it in a clean kitchen towel and place something heavy on top, or use a tofu press if you have one.
Crumble the tofu with your hands until it resembles ground meat.
In a small bowl, whisk together the hoisin sauce, water, soy sauce, vinegar, and sesame oil, if using. Set aside.
Heat the oil in a large skillet over medium-high heat. Add the crumbled tofu and cook for 6–8 minutes, stirring occasionally, until lightly golden and most of the moisture has evaporated. Let it cook undisturbed for 1–2 minutes at a time so it browns properly.
Stir in the garlic, ginger, and the white parts of the green onions. Cook for 1–2 minutes, stirring frequently, until fragrant.
Pour the sauce into the skillet and stir well to coat the tofu. Cook for 2–3 minutes, stirring occasionally, until the sauce thickens slightly and becomes lightly sticky.
Stir in the chopped water chestnuts and cook for 1 more minute, just until heated through.
Spoon the tofu mixture into the lettuce leaves and top with the green parts of the green onions and sesame seeds if desired.
Notes
Store the tofu filling in an airtight container in the refrigerator for up to 4 days. Keep the lettuce leaves separate so they stay crisp.
Let the filling cool completely before freezing it in a freezer-safe container for up to 2 months.
Transfer the filling to the fridge the night before so it can thaw completely.
Reheat the filling in a skillet over medium heat or in the microwave until hot. Add a splash of water if the sauce thickens too much.