Heat sesame oil in a skillet over medium heat. Add fresh ginger, onion and minced garlic. Stir until fragrant.
Add the chopped mushrooms and sauté for 3–4 minutes until softened.
Stir in the Gardein beef and cook until browned and heated through.
Add in hoisin sauce, soy sauce, and rice wine vinegar. Stir to combine.
Mix in the water chestnuts and let the filling simmer for a couple more minutes to absorb the sauce.
Taste and adjust seasoning. Add a drizzle of chili oil if you like it spicy.
Spoon the mixture into lettuce leaves, top with scallions, and serve!