Preheat the oven to 375°F. Then slice red bell peppers in halves, don’t remove theirs steams, so they keep their shape when baked.
Place red bell peppers with the empty face downside and cook for 15 minutes or until it softens. In the meantime, prepare the filling.
Chop chicken into small pieces and set it aside. Then dice the onions and set them aside.
Add 1 tablespoon of olive oil to a large skillet. Once heated, add chopped onion. Stir until the onions have softened and turn translucent. Next, add chicken and stir chicken cooked through.
Season chicken with the homemade fajita seasoning and remove from heat.
Turn peppers over and fill with the cooked chicken filling. Top each pepper with mozzarella cheese.
Bake for another 15 to 20 minutes until peppers are fork-tender and mozzarella has melted.
Serve with diced avocado, fresh lemon juice, and chopped cilantro for garnish.