In a mixing bowl, combine shredded mozzarella, crumbled feta, chopped spinach, diced tomato, and thinly sliced red onion. Sprinkle in black pepper to taste and stir until all ingredients are evenly mixed.

Lay a tortilla flat on a clean surface. Place a quarter of the filling mixture on one half of the tortilla, fold it over to create a half-moon shape, and seal in the filling.

In a non-stick skillet, heat olive oil over medium heat until it shimmers but doesn't smoke.

Carefully place the folded quesadilla in the hot skillet and cook for about 3-4 minutes until the underside is golden brown and crispy.

Once golden, flip the quesadilla and cook the second side for another 3-4 minutes until golden brown.

Remove the quesadilla from the skillet and let it cool slightly before slicing into wedges. Serve warm with sauces or a fresh salad.