Prepare the sauce: In a small bowl, whisk together hoisin sauce, soy sauce, oyster sauce, sesame oil, sriracha, and brown sugar. Set aside.
Cook the pork: Heat oil in a large skillet or wok over medium-high heat. Add ground pork and cook until browned, breaking it apart with a spatula. Drain excess fat if needed.
Add aromatics: Stir in garlic, ginger, and onion. Sauté until fragrant, about 2 minutes.
Add veggies: Add bell pepper, mushrooms, and water chestnuts. Stir-fry for 3–4 minutes until tender.
Add sauce: Pour the prepared hoisin sauce mixture into the pan. Toss well until the pork and vegetables are coated and the sauce slightly thickens.
Finish: Stir in green onions and rice vinegar. Taste and adjust seasoning (add more soy or sriracha if desired).
Serve: Spoon the warm pork mixture into lettuce leaves, sprinkle with peanuts and sesame seeds, and serve immediately.