In a mixing bowl, combine the shredded mozzarella cheese, crumbled feta, chopped spinach, diced tomato, and thinly sliced red onion. Season with black pepper to taste. Stir the mixture until all ingredients are well blended.

Lay one flour tortilla flat on a clean surface, then scoop one-fourth of the filling mixture onto one half of the tortilla. Fold the tortilla over to encompass the filling.

In a large skillet, heat 1 tablespoon of olive oil over medium heat until shimmering, about 1-2 minutes.

Carefully place the filled quesadilla into the hot skillet. Cook for 3-4 minutes on one side until golden brown and crispy.

Flip the quesadilla over and continue cooking for another 3-4 minutes, pressing gently until golden and crispy.

Remove the quesadilla from the skillet and let it cool slightly on a cutting board. Slice it into wedges for easy serving.