In a mixing bowl, combine the shredded mozzarella, crumbled feta, chopped spinach, diced tomato, thinly sliced red onion, and black pepper. Mix all the ingredients until they are well combined.

Lay one flour tortilla flat on a clean surface.

Place one-fourth of the filling mixture on one half of the tortilla.

Fold the tortilla over, creating a half-moon shape that encloses the filling.

Heat a non-stick skillet over medium heat and add the olive oil.

Carefully place the filled tortilla in the skillet and cook for about 3 to 4 minutes, or until golden brown.

Flip the quesadilla and cook for another 3 to 4 minutes until both sides are crispy.

Remove from heat, slice into wedges, and serve warm.