In a mixing bowl, combine shredded mozzarella, crumbled feta, chopped spinach, diced tomato, thinly sliced red onion, and a pinch of black pepper. Mix thoroughly until well combined.

Lay a tortilla flat on a clean surface. Spoon one-fourth of the prepared filling onto half of the tortilla, ensuring an even spread. Fold the other half over the filling.

Heat a non-stick skillet over medium heat and drizzle a tablespoon of olive oil. Allow the oil to heat for 1-2 minutes.

Place the folded quesadilla in the skillet and cook for 3-4 minutes until the bottom is golden brown and crispy.

Carefully flip the quesadilla and cook the other side for an additional 3-4 minutes.

Remove the quesadilla and place it on a cutting board. Allow it to cool for a minute before slicing it into wedges.