In a medium skillet or griddle over medium heat, warm 1 teaspoon of oil. Once the oil is glistening, working with one or two at a time, add the tortillas and sprinkle each with 1/4 cup each of mozzarella, beef, caramelized onions, and Cheddar, then top with another tortilla. Cook until the bottom tortilla is golden brown and the mozzarella is melted, 3 to 4 minutes. Flip the quesadilla and cook until the other tortilla is golden brown and the cheddar is melted, 2 to 3 minutes more. Remove from the skillet and repeat with the remaining ingredients, adding 1 teaspoon oil to the skillet for every quesadilla.☞ TESTER TIP: To keep these quesadillas warm while you finish cooking, pop them on a sheet pan and into a 200°F (95°C) oven.