In a mixing bowl, combine mozzarella cheese, feta cheese, fresh spinach, diced tomato, thinly sliced red onion, and black pepper. Stir gently until well mixed.

Lay one tortilla on a clean surface and scoop one-fourth of the filling mixture onto one half. Spread evenly and fold over to encase the filling.

In a non-stick skillet, heat olive oil over medium heat until shimmering, about 1-2 minutes.

Place the filled tortilla in the skillet and cook for 3-4 minutes until the bottom is golden brown. Flip and cook the other side for an additional 3-4 minutes.

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