In a medium bowl, combine diced large tomato, chopped red onion, fresh spinach, crumbled feta cheese, shredded mozzarella, and a sprinkle of dried oregano. Gently pat the spinach with a paper towel to remove excess moisture, ensuring the filling is not soggy. Mix until well combined, and set aside as you prepare your tortillas.

Lay out your flour tortillas on a clean surface. Divide the filling evenly, placing approximately 1/3 cup of the cheesy Mediterranean mixture on one half of each tortilla. Fold each tortilla over the filling, creating a half-moon shape.

Preheat a non-stick skillet over medium heat for about 2 minutes. Lightly oil the skillet with olive oil.

Carefully place the folded quesadillas in the hot skillet. Cook for 3-4 minutes until the bottoms are golden brown and crisp.

Once golden, use a spatula to carefully flip each quesadilla over. Continue cooking for another 3-4 minutes until they are crispy and the cheese is fully melted.

Let the quesadillas rest for about 1 minute on a cutting board before slicing into halves or triangles.