Prepare the Teriyaki Sauce: In a large bowl, whisk together the soy sauce, hoisin sauce, sesame oil, rice vinegar, minced garlic, grated ginger, and optional red pepper flakes until well combined, then set the sauce aside.
Cook the Ground Chicken: Heat 1 tablespoon of olive oil in a large skillet or Dutch oven over medium heat, then add the ground chicken and immediately begin breaking it apart with a wooden spoon or spatula; cook for 4 to 5 minutes.
Add the Vegetables: Stir the diced red bell pepper and sweet yellow onion into the cooked chicken mixture, then continue cooking for another 4 to 5 minutes.
Finish the Filling: Pour the prepared teriyaki sauce into the skillet and reduce the heat to medium-low; cook the chicken and vegetables in the sauce for 3 minutes so the flavors combine.
Transfer the Filling: Remove the chicken, pepper, and onion mixture from the skillet and place it in a bowl.
Prepare the Lettuce: Rinse the romaine leaves gently under cold water, avoiding hot water because it can cause the lettuce to wilt; pat the leaves dry with paper towels to remove excess moisture.
Assemble the Wraps: Spoon the warm chicken filling into the lettuce leaves, dividing it among the leaves as desired.
Add the Toppings: Finish each lettuce wrap with the julienne carrots, minced cilantro, chopped peanuts, and sliced green onion.
Serve: Serve the lettuce wraps immediately, with Sweet Thai Chili Sauce on the side if desired.
