Prepare the Filling: In a large mixing bowl, combine diced tomatoes, red onion, chopped spinach, crumbled feta cheese, shredded mozzarella, and black pepper. If using fresh spinach, briefly sauté it with minced garlic in a pan until wilted, then press out excess moisture with paper towels before adding to the bowl. Stir in fresh herbs if using.

Assemble Quesadillas: Lay one tortilla flat on your work surface. Place approximately ⅓ of the filling mixture onto one half of the tortilla, leaving a small border around the edges. For extra cheesiness, sprinkle additional mozzarella over the filling. Fold the empty half of the tortilla over the filling, creating a half-moon shape. Gently press down to compact. Repeat with remaining tortillas and filling.

Cook the Quesadillas: Heat 1-2 teaspoons olive oil in a large flat pan or griddle over medium heat. Once hot, carefully place one quesadilla in the pan. Cook for 3-4 minutes per side, until lightly golden brown and cheese is melted. Use a spatula to gently press down on the quesadilla while cooking to help cheese adhere to both sides. Remove and repeat with remaining quesadillas.

Cool and Serve: Remove quesadillas from pan and let cool for 5 minutes before slicing. Using a sharp knife or pizza cutter, slice each quesadilla into 3-4 triangular pieces. Serve warm with your favorite dipping sauces.