In a mixing bowl, combine the shredded mozzarella, crumbled feta, chopped spinach, diced tomato, thinly sliced red onion, and black pepper. Stir until well mixed.

Lay a flour tortilla flat and spoon a quarter of the filling onto one half of the tortilla. Fold the other half over to create a semi-circle.

Heat olive oil in a large skillet over medium heat until shimmering.

Place the quesadilla in the skillet and cook for 3-4 minutes on one side until golden brown.

Flip the quesadilla and cook for another 3-4 minutes until the second side is golden brown and cheese has melted.

Remove from skillet, let cool for a minute, then slice into wedges, serving with salsa or guacamole.