In a large mixing bowl, combine shredded mozzarella, crumbled feta, chopped fresh spinach, diced tomato, sliced red onion, and a sprinkle of black pepper. Gently mix until well incorporated.

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

Heat a non-stick skillet over medium heat, adding olive oil to coat. Place one folded quesadilla in the skillet and cook for 3-4 minutes until golden brown.

Flip the quesadilla and cook the other side for an additional 3-4 minutes until crispy and golden.

Remove from the skillet, let sit for a minute, then slice into wedges and serve warm.