Heat the vegetable oil in a large skillet over medium heat.
Add the diced onion and sauté until translucent, about 3-4 minutes.
Stir in the minced garlic and cook for an additional minute until fragrant.
Add the ground chicken and cook until browned and cooked through, about 7-10 minutes.
Incorporate the diced mushrooms and bell peppers, cooking for another 5 minutes until softened.
Stir in the soy sauce, hoisin sauce, rice vinegar, salt, and pepper. Cook for another 2-3 minutes to let the flavors meld together.
Remove from heat and let cool slightly before assembling.
Wash and separate the lettuce leaves carefully.
Spoon the chicken mixture into the lettuce leaves, garnish with green onions and cilantro, and serve.