In a medium mixing bowl, combine the shredded mozzarella, crumbled feta, chopped spinach, diced tomato, thinly sliced red onion, and black pepper. Stir well to ensure all ingredients blend together.

Take one tortilla and lay it flat on a clean surface. On one half of the tortilla, place one-fourth of the prepared filling and fold the tortilla over to form a semi-circle.

Heat a non-stick skillet over medium heat and drizzle in a little olive oil. Once hot, carefully place the folded quesadilla in the skillet. Cook for about 3-4 minutes until the bottom is golden brown, then flip and repeat.

Once both sides are golden and crispy, remove the quesadilla from the skillet, let it cool for a minute, then slice into wedges and serve warm.