MIX cheese and onion. Spoon about 2 Tbsp. of the cheese mixture onto center of each tortilla; fold tortilla over to completely enclose filling.
SPRAY large skillet with cooking spray. Cook 1 or 2 quesadillas at a time on medium-high heat 2 to 3 min. on each side or until crisp and lightly browned on both sides.
CUT each quesadilla into thirds. Serve warm with the salsa.