In a medium bowl, combine diced tomatoes, finely chopped red onion, fresh spinach, crumbled feta, and shredded mozzarella. Gently mix the ingredients.

Take one flour tortilla and place it on a clean surface. Spoon about a third of your filling mixture onto one half of the tortilla, fold it over, and press down lightly to seal the edges.

In a large skillet, heat 1-2 tablespoons of olive oil over medium heat. Swirl the oil to coat the bottom of the pan.

Carefully place the folded quesadillas in the hot skillet, cooking them for about 3-4 minutes on each side until golden brown and cheese melts.

Once cooked, remove them from the skillet, let cool for a minute, then slice into wedges for serving.