Cook the ground chicken and onion in a skillet over medium heat for 6–8 minutes, breaking it apart until fully cooked.
Add chili powder, cumin, garlic powder, salt, and pepper and cook for another 1–2 minutes.
Warm the tortillas briefly so they are flexible and easy to fold.
Place chicken over half of each tortilla, then add cheese and salsa if using. Fold each tortilla over the filling.
Heat a skillet over medium-low heat and add a little butter or oil.
Cook each quesadilla for 2–3 minutes per side until golden and the cheese is melted.
Transfer to a cutting board and rest for 1–2 minutes.
Cut into wedges and serve with salsa, sour cream, or guacamole.