In a medium mixing bowl, combine the diced ripe tomatoes, finely chopped red onion, and fresh spinach. Gently toss in crumbled feta and shredded mozzarella cheese, followed by a sprinkle of dried oregano. Mix until evenly distributed and set aside.

Lay out three flour tortillas on a clean surface. Spoon approximately 1/3 of the cheesy Mediterranean filling onto one half of each tortilla, spreading it evenly.

Carefully fold each tortilla over the filling, pressing gently to seal the edges.

Heat a skillet over medium heat, adding about 1 tablespoon of oil to coat. Let oil warm for 1-2 minutes.

Place the folded quesadillas into the hot skillet, cooking for 3-4 minutes until the bottoms are golden brown.

Flip each quesadilla over and cook the second side for another 3-4 minutes until toasted and cheese is melted.

Remove the quesadillas from the skillet and let rest for about 1 minute on a cutting board before slicing.

Serve warm, possibly with a side of tzatziki sauce or a cucumber-tomato salad.