Heat a nonstick skillet over medium heat. Spray with nonstick cooking spray and add the bell peppers and onion. Cook the vegetables for 10 minutes.
Add the seasoning ingredients to a small bowl and mix together.
Cut the steak into thin even strips and add it to a bowl. Add the fajita seasoning and stir to combine.
Push the vegetables to the side and add the steak. Cook for 1-2 minutes on each side.
Remove the vegetables and steak from the pan. Wipe the pan clean.
Assemble the quesadillas by adding 2.5 ounces of steak to half of the tortilla. Top it with the vegetables and 2 tablespoons of cheese. (Repeat until all 4 quesadillas are made.)
Spray the pan with cooking spray. Add the filled tortilla and cook for 2-3 minutes or until the tortilla is golden and the cheese is melted.