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

Lay one tortilla flat on a clean surface. Place a generous one-fourth of your filling mixture onto one half of the tortilla and fold the other half over the filling.

In a non-stick skillet, heat olive oil over medium heat until shimmering but not smoking.

Gently place the filled tortilla into the hot skillet. Cook for 3-4 minutes until golden brown, then flip and cook the other side for another 3-4 minutes.

Remove the quesadilla from the skillet, let it cool for a minute, and slice into wedges for serving.