In a mixing bowl, combine shredded mozzarella, crumbled feta, chopped fresh spinach, and diced small tomato. Mix in thinly sliced red onion and black pepper until evenly blended.

Lay out one flour tortilla flat and spoon one-fourth of the filling mixture onto half of the tortilla. Fold the other half over and press down gently.

Place a non-stick skillet over medium heat and warm for about 2 minutes. Add olive oil, swirling to coat the pan.

Carefully place the first folded quesadilla in the skillet. Cook for 3-4 minutes, pressing down with a spatula. Flip when golden brown and cook for another 3-4 minutes.

Remove quesadilla from the skillet, let cool on a cutting board, and slice into wedges. Serve hot with salsa or Greek yogurt.