In a medium bowl, combine the chopped spinach, red onion, diced tomato if using, crumbled feta, and shredded mozzarella. Add the minced garlic, dried oregano, and olive oil, then season lightly with salt and pepper and mix until everything is evenly coated.

Lay the flour tortillas on a flat surface. Divide the spinach and cheese mixture into four portions and spread each portion over one half of a tortilla, leaving a small border around the edge. Fold the empty half of the tortilla over the filling to form a half moon shape.

Heat a large skillet over medium heat and add a thin layer of olive oil or butter. Place one or two folded tortillas in the pan and cook for about 3 to 4 minutes per side, until the tortillas are golden and crisp and the cheese inside is fully melted.

Transfer the cooked quesadillas to a cutting board and let them rest for 1 minute. Slice each quesadilla into wedges using a sharp knife or pizza cutter, then serve hot with your favorite simple toppings, such as plain Greek yogurt, lemon wedges, or salsa.