In a large bowl, toss the sliced onion and bell peppers with half of the fajita seasoning and 1 tablespoon of olive oil.

Heat a large cast iron skillet over medium-high heat. Add the vegetables and cook until soft and slightly charred, about 10–12 minutes. Remove from the pan and set aside.

In the same bowl, toss the sliced chicken with the remaining seasoning and 1 tablespoon of oil.

Add the chicken to the hot skillet and cook for 6–8 minutes or until cooked through, flipping halfway through.

Return the cooked vegetables to the pan, squeeze in the lime juice, and stir to combine. Remove from heat.

Lay one tortilla flat on a clean surface. Spoon about ¼ of the fajita mixture over half of the tortilla, sprinkle with cheese, and fold over to close.

Heat a clean skillet or griddle over medium heat and melt ½ tablespoon butter.

Place the quesadilla in the skillet and cook until golden brown and crisp, about 2–3 minutes per side. Remove from the pan and repeat with remaining quesadillas, adding more butter as needed.

Slice quesadillas in half and serve warm, topped with fresh cilantro.