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

Take a flour tortilla and lay it flat. Spoon one-fourth of the filling mixture onto one half and fold the tortilla over to cover the filling.

Place a skillet over medium heat and add one tablespoon of olive oil. Allow the oil to heat until it shimmers.

Carefully place the folded quesadilla into the hot skillet. Cook for 3-4 minutes, pressing down gently with a spatula.

Once the first side is golden, carefully flip the quesadilla and cook the other side for an additional 3-4 minutes.

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