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

Once the onions and garlic are ready, add in the fresh spinach leaves to the skillet. Cook this mixture for about 2 minutes, stirring until the spinach wilts down significantly. Season with a pinch of salt and black pepper.

Take a large flour tortilla and lay it flat. Spoon a quarter of the spinach mixture over half of the tortilla, then sprinkle crumbled feta and shredded mozzarella over the spinach. Fold the tortilla in half.

In the same skillet, melt half of the remaining butter over medium heat. Place the folded quesadilla in the skillet and cook for 3-4 minutes until golden brown, then flip and cook the other side.

Remove the quesadilla from the skillet and let it rest for 1 minute before slicing it into wedges. Serve immediately.