Heat a large cast-iron skillet or a large pan on medium-high heat. When the pan is hot (sprinkle a few drops of water in the pan, if it sizzles the pan is ready) add the oil or water and spread evenly in the pan. Add the onion slices and spread them evenly in the pan. Stir frequently until they begin to caramelize or their edges turn brown. Approximately 5 to 8 minutes. Add the spinach and spread evenly in the pan. Stir frequently. Cook until the spinach is wilted. Approximately 5 minutes. Salt and pepper to taste.
Remove the onion and spinach mixture from the pan.
Lay the tortillas flat and spread a half inch to an inch thick layer of hummus on half of the tortilla. Put an even layer of the onion and spinach mixture on top of the hummus and fold the unfilled layer over the filled part of the tortilla. Transfer to the hot skillet and cook until the tortilla begins to crisp up and turn a light golden brown. Carefully flip the quesadilla and cook until that side turns a light golden brown. Approximately 3 to 5 minutes per side depending on how hot your pan is.
Cut into triangles and enjoy!