In a large mixing bowl, combine shredded mozzarella, crumbled feta, fresh spinach, diced tomato, finely chopped red onion, and black pepper. Mix until well combined.

On a clean surface, place one tortilla and spoon a quarter of the filling onto one half. Fold the tortilla over to create a half-moon shape.

Preheat a non-stick skillet over medium heat for about 2 minutes. Drizzle in a tablespoon of olive oil.

Carefully place one folded quesadilla in the skillet and cook for 3-4 minutes until golden and crispy.

Using a spatula, flip the quesadilla and cook for another 3-4 minutes until browned and the cheese is melted.

Transfer the quesadilla to a cutting board, let cool for a minute, and slice into wedges.