1. Stack tortillas; microwave at HIGH 1 minute. Leave in microwave to keep warm while preparing filling.
2. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add mushrooms; sauté 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.
3. Spoon about 1/3 cup bean mixture onto each tortilla. Sprinkle evenly with cheese and onions. Fold tortillas in half.
4. 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.