In a mixing bowl, combine shredded mozzarella, crumbled feta, fresh spinach, diced tomato, sliced red onion, and a sprinkle of black pepper. Mix until evenly distributed. Set aside.

Lay one tortilla flat on a clean surface, scoop a quarter of the filling onto one half of the tortilla, and fold over. Securely tuck the filling inside.

Heat a non-stick skillet over medium heat, add olive oil, and place the quesadilla in the skillet. Cook for 3-4 minutes until golden brown and crispy.

Carefully flip the quesadilla and cook the other side for another 3-4 minutes until both sides are golden brown and cheese is melted.

Remove from skillet, let cool slightly, slice into wedges, and serve warm with a salad or yogurt dip.