Stack tortillas; microwave at HIGH 1 minute. Leave in microwave to keep warm while preparing filling.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray.
Add mushrooms; saut 2 minutes or until tender.
Add vinaigrette, beans, and pimiento; cook 1 to 2 minutes or until liquid evaporates, stirring constantly. Mash bean mixture slightly with a potato masher.
Spoon about 1/3 cup bean mixture onto each tortilla.
Sprinkle evenly with cheese and onions. Fold tortillas in half.
Wipe skillet with paper towels; heat over medium heat. Coat pan with cooking spray.
Place 2 quesadillas in pan; cook 2 to 3 minutes on each side or until golden and cheese melts. Repeat procedure with remaining 2 quesadillas.
Cut each quesadilla into 3 wedges.
Serve immediately with salsa and sour cream, if desired.