Lay a tortilla on your work surface. Scatter half the cheese, then half the beans over one side. Fold the tortilla in half over the filling. Repeat with the second tortilla.
Warm a wide, heavy pan over medium heat. Pour in 1/2 teaspoon oil and tilt the pan to spread the oil around.
Put a quesadilla in the pan and cook until browned underneath, about 1 1/2 minutes. Flip it, then cook it until it's browned on the other side and the cheese is melted, about 1 1/2 minutes longer. Transfer it to a plate and repeat with the second quesadilla.
Cut each quesadilla into wedges and serve immediately with salsa, sour cream, and guacamole.