This easy quesadilla recipe uses canned cooked chicken to save time. You can also use leftover cooked chicken or turkey if you have it. Dice the vegetables up to 1 day ahead for even speedier prep.
Heat oil in a medium skillet over medium heat. Add onion, bell pepper, and zucchini; cook, stirring often, until the vegetables are tender, 3 to 4 minutes. Add chicken and corn; cook until heated through, about 1 minute. Stir in cilantro, if using. Transfer the vegetables to a small bowl. Wash and dry the skillet.
Place tortilla on a cutting board. Sprinkle 1 Tbsp. cheese over half the tortilla, leaving a 1/2-inch border. Top with the vegetable mixture and the remaining 2 Tbsp. cheese; fold the tortilla in half.
Heat the skillet over medium heat. Add the quesadilla and cook until the tortilla is browned and the cheese has started to melt, about 2 minutes per side. Cut into 3 wedges to serve.