In a large bowl, add the ground turkey, spinach, zucchini, feta, red onion, parsley, lemon zest, garlic powder, oregano, salt, pepper, and crushed red pepper flakes. Mix with your hands to combine.
Form the mixture into four ½-inch-thick patties.
Preheat the grill (or grill pan) to medium heat and brush with oil. Grill the burgers until cooked through, about 5 to 6 minutes per side. A turkey burger is fully cooked when the center of the patty reaches an internal temperature of 165° F.
Place the burgers on the toasted buns and top with tzatziki, lettuce, tomato, and red onion. Serve immediately.
