Heat the oil in a pan over medium-high heat. Once hot, add onion and sauté until the onion softens and becomes translucent.
Add the ground chicken and cook, stirring occasionally to break up the meat, until browned. About 6-7 minutes.
Add the bell peppers and water chestnuts and cook on medium heat for 2-3 minutes, or until the peppers soften a little.
In a medium bowl, whisk together all of the sauce ingredients: soy sauce, hoisin sauce, sesame oil, minced garlic, minced ginger, peanut butter, and sriracha.
Pour the sauce into the chicken mixture and let it simmer on low heat until the chicken and veggies are well coated with the sauce. Season with salt and pepper.
To serve, spoon 2-3 tablespoons of the chicken mixture into the center of the lettuce leaf and top it with some chopped spring onions and sesame seeds.