Combine the shredded mozzarella, crumbled feta, chopped spinach, diced tomato, sliced red onion, and black pepper in a bowl and stir everything together thoroughly.
Place one tortilla on a flat surface and spread a quarter of the mixture over half of the tortilla; then fold the tortilla over to cover the filling.
Warm a non-stick skillet on medium heat and drizzle the olive oil.
Set the filled tortilla into the skillet and let it cook for about 3 to 4 minutes until the underside is browned and crisp.
Flip the quesadilla carefully and cook the other side for another 3 to 4 minutes until it’s golden and crunchy.
Take it off the heat, slice into wedges, and enjoy warm.