In a medium skillet, add a little water (or oil, if using) over medium heat. Add onion; cook for 3-4 minutes, until they start to become translucent. Add bell pepper; cook for 5 minutes. Add tomato and corn; cook for 5 more minutes. Add spinach; cook until wilted, about 1 minute.
Place a tortilla flat. On one half of the tortilla, add half of the hummus; spread evenly. Add half of the vegetables and 1/4 cup black beans on top of the hummus.
Fold the tortilla in half.
In the same skillet, add the quesadilla. Over medium heat, cook for 2-3 minutes on each side, until slightly crispy.
Remove; cut into 4 even pieces.
Repeat with the other tortilla and the rest of the fillings.
Serve with mashed avocado, salsa, and fresh cilantro.