Place a flat griddle or large nonstick skillet over medium heat. Let it warm up for about 2 minutes.
Lay one flour tortilla flat on a clean surface. On one half of the tortilla, spread 2 tablespoons of marinara or pizza sauce.
Sprinkle about half of your shredded mozzarella cheese over the sauce. Add your choice of toppings—pepperoni, sautéed veggies, sausage, or whatever your pizza-loving heart desires. Then, top with the remaining mozzarella cheese for extra gooeyness.
Fold the tortilla over to create a half-moon shape. Spray the skillet with cooking spray (or brush lightly with oil), then place the folded quesadilla onto the hot surface.
Cook for 2–3 minutes per side, or until both sides are golden brown and the cheese inside has melted completely.
Transfer the quesadilla to a cutting board. Let it cool for a couple of minutes, then slice into triangles using a sharp knife or pizza cutter.