Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the chopped onions and minced garlic, sautéing them for about 2-3 minutes until they become fragrant and the onions are translucent.

Once the onions are translucent, add about 4 cups of fresh spinach to the skillet. Stir continuously for about 2 minutes or until the spinach has wilted down considerably. Season it with salt and black pepper to taste. Remove the skillet from heat and set aside the spinach mixture to cool slightly.

On a clean, flat surface, place one large flour tortilla. Spoon a quarter of the spinach mixture onto one half of the tortilla, then top it with crumbled feta and shredded mozzarella cheese.

Carefully fold the tortilla in half, pressing it gently to seal in the delicious filling. Repeat this process with the remaining tortillas and spinach mixture.

In the same skillet, melt half of a tablespoon of butter over medium heat. Place the first quesadilla in the skillet, cooking for about 3-4 minutes or until the bottom side is golden brown and crispy. Flip it over gently and cook the other side for an additional 3-4 minutes until the cheese is melted.

Once cooked, remove the quesadilla from the skillet and let it rest for about 1 minute on a cutting board. Slice the quesadilla into wedges and serve hot.