Heat olive oil in a skillet over medium-high heat. Add sliced chicken and cook until browned and cooked through, about 5-7 minutes.

Add sliced bell pepper and onion to the skillet. Sprinkle with fajita seasoning and cook until vegetables are tender, about 5 minutes.

Place a tortilla in the skillet over medium heat. Sprinkle half of the cheese on one half of the tortilla.

Add a portion of the chicken fajita mixture on top of the cheese, then sprinkle with remaining cheese. Fold the tortilla in half.

Cook until the tortilla is golden brown and the cheese is melted, about 3-4 minutes per side. Repeat with remaining tortillas.

Cut quesadillas into wedges and serve with sour cream and salsa.