In a mixing bowl, combine the shredded mozzarella, crumbled feta, chopped spinach, diced tomato, thinly sliced red onion, and black pepper. Gently toss until well mixed.

Lay one flour tortilla flat and place one-fourth of the filling on one half. Fold the other half over the filling.

Preheat the skillet over medium heat for about 2 minutes. Add a tablespoon of olive oil.

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

Remove the quesadilla from the skillet and let it cool for a minute before slicing into wedges.