Sauté the Aromatics: Heat the 1 tbsp of olive oil in a large frying pan or skillet over a medium heat. Add the finely diced red onion and cook for 3-4 minutes, until it begins to soften and turn translucent. Add the minced garlic and cook for another minute until fragrant, being careful not to let it burn.

Wilt the Spinach: Add the fresh spinach to the pan. You may need to do this in batches. Stir it with the onion and garlic as it wilts down, which should only take about 2-3 minutes.

Drain the Spinach: Transfer the cooked spinach mixture to a fine-mesh sieve. Using the back of a wooden spoon or a spatula, press down firmly on the spinach to squeeze out as much excess liquid as possible. This step is vital for a crisp quesadilla. Let it cool for a few minutes.

Combine the Filling: In a medium mixing bowl, combine the squeezed spinach mixture, crumbled feta cheese, grated mozzarella, dried oregano, and fresh lemon juice. Season with a generous amount of black pepper. Mix everything together until well combined.

Assemble the Quesadillas: Lay a tortilla flat on a clean surface. Spread one quarter of the spinach and feta mixture evenly over one half of the tortilla. Fold the other half over to create a semi-circle. Repeat with the remaining tortillas and filling.

Cook the Quesadillas: Wipe out your frying pan and return it to a medium heat. Melt about half a tablespoon of butter or add a little oil. Carefully place one or two quesadillas in the pan (don't overcrowd it). Cook for 2-4 minutes on the first side, until the bottom is golden brown and crisp. I find that pressing down gently with a spatula helps it cook evenly.

Flip and Finish: Flip the quesadilla and cook for another 2-3 minutes on the other side, until it's also golden brown and the cheese inside is completely melted and gooey.

Serve: Remove the cooked quesadilla from the pan and place it on a cutting board. Let it rest for a minute before slicing it into wedges. Repeat the cooking process with the remaining quesadillas, adding more butter to the pan as needed. Serve immediately.