In a medium bowl, combine diced ripe tomatoes, chopped red onion, fresh spinach, crumbled feta cheese, shredded mozzarella, and dried oregano. Mix well and let the flavors mingle.

On a clean surface, lay out three flour tortillas. Distribute about 1/3 cup of the filling onto one half of each tortilla, fold over, and press lightly.

Heat a large skillet over medium heat with a small drizzle of oil for 1-2 minutes until shimmering.

Place the folded quesadillas in the skillet and cook for 3-4 minutes until golden brown, then flip and cook for another 3-4 minutes until cheese is melted.

Transfer to a cutting board, let rest for 1 minute, then slice into halves or triangles and serve warm.