In a mixing bowl, combine the shredded mozzarella, crumbled feta, fresh spinach, diced tomatoes, and thinly sliced red onion. Add a pinch of black pepper for flavor, and mix everything thoroughly until well combined.

Lay one flour tortilla flat on a clean surface. Spoon one-fourth of the filling mixture onto one half of the tortilla. Gently fold the other half over the filling, creating a half-moon shape. Press down slightly to secure the tortilla.

Heat a large skillet or griddle over medium heat, adding about a tablespoon of olive oil to the pan. Allow the oil to warm for a minute until shimmering.

Carefully place the filled tortilla in the hot skillet. Cook each quesadilla for 3-4 minutes on one side, pressing gently with a spatula to ensure even cooking.

After 3-4 minutes, flip the quesadilla over using your spatula. Cook for an additional 3-4 minutes on the other side until it's equally crispy and the cheese is fully melted.

Once cooked, remove the quesadilla from the skillet and let it cool for a minute on a cutting board. Slice each quesadilla into wedges for easy sharing.