Make the quesadilla(s) one at a time: heat a 10-inch skillet over medium heat. Warm one tortilla for about 30 seconds, flipping halfway. Flip once more, and sprinkle one-half of the tortilla with about half of the shredded cheese. Cover the cheese with chickpeas, tomatoes, olives, and red onion. Sprinkle the remaining cheese over the other fillings and fold over the empty side of the tortilla to enclose the fillings.