Combine hoisin sauce, soy sauce, rice vinegar and sesame oil in a small bowl. Whisk well and set aside.
Heat a large nonstick pan on medium-high heat. Crumble the tofu and add to the pan. Cook as you crumble further until the mixture starts to turn lightly golden, about 10 minutes.
Add the ginger and garlic; cook till fragrant, about 1~2 minutes.
Next add the mushrooms and cook till the mushrooms start to cook down and the mixture is not wet anymore.
Stir in the water chestnuts, crushed red pepper flakes and half of the chopped green onions. Cook for 1~2 minutes.
Pour the sauce into the pan and stir to coat. Cook till the sauce starts to bubble and is warmed through, about 1~2 minutes.
Spoon the mixture into individual lettuce leaves. Top with grated carrot, chopped peanuts and remaining chopped scallions. Serve right away.