In a mixing bowl, combine the mozzarella cheese, crumbled feta, fresh spinach, diced tomato, and thinly sliced red onion. Season with black pepper to taste and mix well.

On a clean surface, place one tortilla and spread one-fourth of the filling mixture over one half of the tortilla. Fold the tortilla to create a crescent shape and press down gently to seal.

Heat a non-stick skillet over medium heat and add a drizzle of olive oil, swirling to coat the bottom evenly.

Place the folded quesadilla in the hot skillet and cook for about 3-4 minutes until the bottom is golden brown. Gently press down with a spatula.

Carefully flip the quesadilla and cook for an additional 3-4 minutes on the other side until golden brown and crispy.

Remove from skillet, slice into wedges, and serve warm with dipping sauce or salad.