In a small bowl, whisk together soy sauce, rice vinegar, hoisin sauce, sesame oil, Sriracha, and corn starch. Set aside.
Heat avocado oil in a large skillet over medium high heat. Add garlic and sauté for 30 seconds or until fragrant. Add chicken and season with salt and pepper. Cook chicken, stirring frequently, until golden brown and almost cooked through; about 4-5 minutes.
Add the red bell pepper and cook for an additional 2 minutes or until chicken is fully cooked. Pour soy sauce mixture over chicken and stir to coat. Bring mixture to a simmer and allow sauce to thicken slightly; about 2 minutes. Season to taste with salt and pepper. Remove from heat.
Spoon chicken mixture into lettuce leaves and top with fresh bell pepper, carrots, cabbage, and scallions. Squeeze fresh lime juice over the top and sprinkle with crushed red pepper flakes if desired.