Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add onion; sauté 5 minutes or until tender. Remove from pan. Wipe pan with a paper towel.
Coat pan with cooking spray; place over medium heat until hot. Place 1 tortilla in pan. Cook 1 minute or until bottom of tortilla is golden. Sprinkle one-fourth each of onion, cheese, and cumin over half of tortilla. Fold tortilla in half. Cook tortilla 1 minute on each side or until golden and cheese melts. Repeat procedure with remaining tortillas, onion, cheese, and cumin.
To serve, top each quesadilla with 2 tablespoons salsa and 2 tablespoons sour cream.