In a mixing bowl, combine the shredded mozzarella, crumbled feta, chopped spinach, diced tomato, and thinly sliced red onion. Season with black pepper and mix well.

Lay a tortilla flat on a clean surface. Place a generous quarter of the filling mixture on one half, fold over, and press down gently.

Heat a skillet over medium heat, add olive oil, and allow it to shimmer.

Carefully place the filled tortilla into the skillet. Cook for about 3-4 minutes until golden brown and crisp.

Flip the quesadilla using a spatula and cook for an additional 3-4 minutes on the other side.

Remove from skillet, let cool for a couple of minutes, and slice into wedges.