In a medium mixing bowl, combine the shredded mozzarella, crumbled feta, chopped spinach, diced tomato, thinly sliced red onion, and a sprinkle of black pepper. Stir until fully integrated.

Take a tortilla and lay it flat, then spoon a quarter of the filling onto one half of the tortilla and fold it over.

Heat the non-stick skillet over medium heat, add olive oil, and cook the folded quesadilla for 3-4 minutes per side until golden brown.

Once crispy, remove from skillet, slice into wedges, and serve warm.