Step 1: Cook the Ground Beef. Place a large skillet over medium-high heat. Add the ground beef and diced onions. Cook while breaking the meat apart with a spatula until the beef is fully browned and the onions are soft. Drain any excess fat from the pan.
Step 2: Season the Meat. Stir in the minced garlic and cook for 1 minute until fragrant. Add the taco seasoning and water. Simmer for 2 to 3 minutes until the liquid has thickened and the beef is well-coated. Remove from heat and set aside.
Step 3: Assemble the Quesadillas. Lay a tortilla flat on a clean surface. Sprinkle a thin layer of cheese over one half of the tortilla. Top the cheese with a generous spoonful of the cooked beef mixture, then add another layer of cheese on top of the beef. Fold the tortilla in half to create a half-moon shape.
Step 4: Prepare for Air Frying. Lightly spray the outside of each assembled quesadilla with olive oil or brush with a small amount of melted butter. This ensures the tortilla becomes golden and crispy.
Step 5: Air Fry the Quesadillas. Place the quesadillas in the air fryer basket in a single layer. You may need to cook in batches depending on the size of your air fryer. Cook at 370°F (185°C) for 5 to 6 minutes, flipping halfway through, until the tortillas are golden brown and the cheese is completely melted.