Try different cheese and apple combinations to find your favorite.
Pour 1 tsp oil in a non-stick pan. Place one tortilla in pan and add cheeses, apples and caramelized onions (optional). Top with second tortilla and spread with 1 tsp oil.
Sauté on medium high heat until cheese starts to melt and bottom tortilla becomes browned. Carefully flip to other side a sauté until tortilla is browned and cheese is completely melted.
Place finished quesadilla on a cutting board and slice into wedges. Serve warm.