In a mixing bowl, combine mozzarella cheese, feta cheese, chopped spinach, diced tomatoes, thinly sliced red onion, and black pepper. Stir until all ingredients are well mixed to create a colorful and flavorful filling.

Lay one tortilla flat on a clean surface or plate. Place one-fourth of the filling mixture onto half of the tortilla to ensure even distribution of flavors when folded.

Carefully fold the tortilla over to cover the filling completely. This will create a half-moon shape filled with your delicious mixture.

Heat a large skillet or griddle over medium heat. Pour in olive oil and allow it to heat up until shimmering; this will help achieve a crispy exterior on your quesadilla.

Place the filled tortilla in the heated pan and cook for about 3-4 minutes per side. Press gently with a spatula to ensure even melting of cheeses and browning of the tortilla.

Once cooked to golden perfection on both sides, remove from heat and let cool slightly. Slice into wedges before serving warm for an enjoyable eating experience!