Stir together refried beans and drained tomatoes in medium bowl. Spread 1/3 cup bean mixture over half of each tortilla. Top each with 1/4 cup chicken and 1/4 cup cheese. Fold each tortilla in half to cover filling.
Spray large skillet with cooking spray; heat over medium heat 1 minute. Place 2 quesadillas side-by-side in skillet. Cook 5 minutes, turning once, or until golden brown on each side and cheese melts. Repeat with remaining quesadillas.
Cut each quesadilla into 4 wedges. Serve with onions and sour cream, if desired.