First, prepare sauce by combining first 8 ingredients and whisking until smooth.
Heat olive oil in nonstick skillet or wok over medium heat, add in diced onion and cook for 3-4 minutes, or until slightly softened and transparent. Add in crumbled tofu and saute another 3-5 minutes, or until some parts get crispy and browned.
Add in sauce, water chestnuts and green onions, stir to combine, and heat through.
Rinse and separate lettuce leaves, spoon in filling, and serve with additional soy sauce and sriracha if desired