In a medium bowl, combine the diced tomato, finely chopped red onion, and spinach. Add the crumbled feta and shredded mozzarella cheese along with the dried oregano. Mix gently until everything is well distributed, ensuring to squeeze the spinach gently to remove any excess moisture.

On a clean surface, lay out the three flour tortillas. Take about one-third of the prepared filling and place it on one half of each tortilla. Gently fold the other half over to enclose the filling, pressing down lightly to secure it.

Heat a skillet over medium heat and add just a splash of oil to prevent sticking. Carefully place each folded quesadilla in the skillet, cooking for 3–4 minutes or until golden brown on the bottom. Flip and cook the other side for another 3–4 minutes, until the cheese is melted and the tortillas are crispy.

Once cooked, remove the quesadillas from the skillet and allow them to rest on a cutting board for about one minute. Slice the cheesy Mediterranean quesadilla into wedges and serve warm.