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

Lay one flour tortilla flat on a surface. Add one-fourth of the filling mixture onto one half of the tortilla and fold over to cover.

Place a skillet over medium heat, add olive oil, and allow it to heat up until shimmering.

Once the oil is hot, place the assembled quesadilla in the skillet. Cook for about 3-4 minutes until golden brown.

Flip the quesadilla and cook for an additional 3-4 minutes until both sides are golden and the cheese has melted.

Remove the quesadilla from the skillet, let cool slightly, and cut into wedges.