Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add onion; sauté 4 minutes. Add spinach; sauté 1 minute or until spinach is wilted. Remove from pan. Wipe pan with a paper towel.
Coat pan with cooking spray; place over medium heat until hot. Place one tortilla in pan. Cook 1 minute or until bottom of tortilla is golden. Sprinkle one-fourth each of spinach mixture, beans, and cheese over one side of tortilla. Fold tortilla in half. Cook tortilla 1 minute on each side or until golden and cheese melts. Repeat procedure with remaining tortillas, spinach mixture, beans, and cheese.