Heat olive oil in a large skillet or pan over medium heat. Add garlic and onion, sauté until fragrant and onions turn translucent.
Add ground chicken to the skillet, breaking it up with a spatula. Cook until chicken is no longer pink, stirring occasionally.
Stir in diced red bell pepper and shiitake mushrooms. Cook for an additional 3-4 minutes until vegetables start to soften.
In a small bowl, mix together soy sauce, hoisin sauce, sesame oil, and sriracha. Pour this sauce over the chicken and vegetable mixture in the skillet. Stir well to combine.
Season with salt and pepper to taste. Cook for another 2-3 minutes, allowing flavors to meld together. Remove from heat.
Spoon the chicken mixture into individual lettuce leaves, creating cups. Garnish with optional toppings like chopped green onions, sesame seeds, or cilantro.