Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the chopped onions and minced garlic, stirring frequently for about 2-3 minutes until the onions become translucent and fragrant.

Add the fresh spinach leaves to the skillet, cooking for an additional 2 minutes or until they are completely wilted. Stir in salt and black pepper to taste, and set aside the spinach filling.

On a clean surface, take one large flour tortilla and spread one-quarter of the cooked spinach mixture over half of the tortilla. Sprinkle crumbled feta and shredded mozzarella cheese on top.

Fold the tortilla over the filling to create a half-moon shape, pressing down gently to secure the edges. Repeat with the remaining tortillas.

Melt 1 tablespoon of butter in the skillet. Place the folded quesadillas in the skillet, cooking for about 3-4 minutes per side until golden brown and the cheese is bubbling.

Remove the quesadillas, let them rest for about a minute, then slice into wedges and serve warm with salsa or sour cream.