Heat avocado oil in a large skillet over medium heat.
Add chopped onion and cook until translucent, about 3-4 minutes.
Add ground chicken and cook until browned, breaking it apart with a spoon.
Stir in diced tomatoes with green chiles, black beans, sweet corn, and fajita seasoning.
Cook for another 5-7 minutes, allowing flavors to combine and mixture to heat through.
Remove from heat and stir in fresh cilantro if using.
Wash and dry lettuce leaves.
Spoon the chicken mixture into each lettuce leaf and serve immediately or refrigerate for meal prep.