Add all of the tzatziki ingredients to a small bowl. Mix together, cover, and refrigerate.
Add the spinach, sun dried tomatoes, reduced-fat feta cheese, chopped red onions, and part-skim mozzarella cheese to a medium bowl. Stir all of the ingredients together.
Lay one tortilla flat on a clean surface. Evenly distribute half of the chopped spinach mixture across one half of the tortilla. Fold the tortilla over to create a half-moon shape. Gently press down on the folded tortilla to help keep the filling in place.
Repeat the assembly process for the second tortilla.
Preheat a skillet over medium heat. Lightly coat the skillet with non-stick spray.
Once the skillet is hot, place the folded quesadilla in the skillet. Cook 3-4 minutes on one side, flip it and cook for an additional 3-4 minutes on the other side.
Press down gently on the quesadilla with the spatula as it cooks to help the cheese bind the filling together. Once cooked, transfer to a cutting board and let sit for a minute to allow the cheese to set slightly. Use a sharp knife or pizza cutter to slice them into wedges.