To make caramelized onions: Add olive oil and butter in a thick-bottomed pan over a medium high heat until the oil is shimmering. Add the onion slices and stir to coat. Spread the onions evenly and let cook, stirring occasionally. Reduce the heat to medium low if the onions are burning or drying out. After 10 minutes, sprinkle salt and sugar. Add a little water to the pan if the onions are drying out. Stir every few minutes (not too often - they won't brown), letting them to cook for another 30-40 minutes. Add a little more olive oil if they burn too quickly. As the onions cook down, scrape the pan every minute if it's necessary. Continue to cook and scrape until the onions are a deep brown color. Lastly, add balsamic vinegar to deglaze the pan.
Preheat oven to 220°C. Line a baking sheet with parchment paper and grease it with olive oil.
On half of a flour tortilla, layer 30g cheese, a handful of spinach, caramelized onions, a few apple slices and another 30g cheese.
Fold in half and brush with olive oil. Place the filled apple-cheese tortillas on the parchment lined baking sheet.
Repeat with each of the remaining tortillas. Bake the cheesy apple and spinach quesadillas until they're a little bit golden and the cheese is melted, about 7-10 minutes.