In a mixing bowl, combine shredded mozzarella, crumbled feta, chopped fresh spinach, diced tomato, and sliced red onion. Season with black pepper. Stir until well combined.

Lay a tortilla flat and spoon one-fourth of the filling onto half of it. Fold the tortilla over to create a half-moon shape.

Heat olive oil in a large skillet over medium heat. Place the folded quesadilla in the skillet and cook for about 3-4 minutes until golden-brown.

Flip the quesadilla and cook for another 3-4 minutes until both sides are crispy and the cheese has melted.

Remove from skillet, let cool for a minute, and slice into wedges. Serve warm.