In a large mixing bowl, combine the spinach, turkey, cottage cheese, feta cheese, oregano, egg, cereal crumbs, and pepper. Mix well.
Form the burgers into 6 patties of equal size.
Coat a large grill pan (or any large pan you like) with cooking spray and set it over medium-high heat.
Add the turkey burgers and cook until browned on both sides, about 6 to 8 minutes per side. An instant-read thermometer inserted in the center of the patties should register 165 degrees F (74 degrees C).
Place cooked turkey burgers on buns and top each with 2 tablespoons Tzatziki sauce. Add lettuce, tomatoes, and onions, if desired.