Preheat oven to 350°F. Heat oil in large skillet on medium-high heat. Add chicken, onion and garlic salt; cook and stir 7 to 8 minutes or until chicken is cooked through.
Place 3 of the tortillas on an ungreased baking sheet. Spread 1/3 of the chicken mixture and 1/2 cup of the cheese evenly on each tortilla. Top with remaining tortillas. Lightly brush tops of tortillas with oil. Sprinkle evenly with remaining cheese.
Bake 5 minutes or until cheese is melted and quesadillas are heated through. Cut into wedges. Serve warm.