In a large bowl, combine the white beans, spinach, red onion, and cheese.
Lay out the tortillas, and divide the bean mixture evenly among them, spreading the mixture on one half of each tortilla.
Fold the tortillas in half, and press down lightly to seal.
In a large nonstick skillet, heat a bit of olive oil over medium heat.
Add the quesadillas to the skillet, and cook for 2-3 minutes on each side, or until the tortillas are golden brown and the cheese is melted.
Serve hot, with salsa, sour cream, and guacamole on the side.