In a mixing bowl, combine mozzarella cheese, feta cheese, chopped spinach, diced tomato, sliced red onion, and black pepper. Mix until well incorporated.

Lay one tortilla flat on a clean surface. Scoop a generous portion of the filling onto one half and fold the other half over.

Heat a non-stick skillet over medium heat. Add olive oil and once shimmering, place the folded quesadilla in the skillet. Cook for about 3-4 minutes.

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

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