In a large skillet, heat 3 tablespoons of avocado oil or olive oil over medium heat. Once hot, add the thinly sliced onions and sprinkle them with 1/2 teaspoon of sea salt. Stir occasionally, allowing the onions to cook slowly for about 15-20 minutes until they are golden brown and caramelized.
Once the onions are caramelized, add the baby spinach to the skillet. Cook for another 2-3 minutes until the spinach is wilted but still vibrant green.
On a clean surface, lay out one large flour tortilla. Sprinkle a generous amount of grated Monterey Jack cheese over half of the tortilla. Next, layer on some of the caramelized onion and spinach mixture, followed by slices of avocado. Top it all off with a little more cheese before folding the tortilla in half.
In the same skillet, add a little more oil if necessary, and place the folded quesadilla in the pan. Cook for about 3-4 minutes on one side until golden brown and crispy. Carefully flip the quesadilla and cook for another 3-4 minutes on the other side.
Once cooked, remove the quesadilla from the skillet and let it cool for a minute. Use a sharp knife or pizza cutter to slice it into wedges. Serve warm with your favorite dipping sauces.