Place a large skillet coated with cooking spray over medium-high heat until hot. Add butter, spinach, and mushrooms; sauté until mushrooms are tender. Remove from heat.
Spread each of 5 tortillas with 1 tablespoon cream cheese; sprinkle evenly with spinach mixture, beans, and cheese. Top each with a tortilla.
Recoat pan with cooking spray; place over medium-high heat until hot. Carefully place 1 quesadilla in pan; cook 2 minutes on each side or until golden, pressing down with a spatula. Repeat procedure with remaining quesadillas, coating pan with cooking spray before adding each. Serve immediately with sour cream.