Squeeze as much moisture as possible from the thawed spinach.
In a bowl, combine the ground turkey with the spinach, grated onion, sun-dried tomatoes, egg, breadcrumbs, feta cheese, oregano, garlic, salt and pepper (to taste). Stir everything until it is evenly combined.
Shape the turkey mixture into 4 equal patties.
Cook the burgers on a stovetop grill, in a nonstick skillet, or over an open flame for about 5-7 minutes per side over medium-high heat.
Serve burgers on buns and top with Tzatziki sauce, lettuce and tomato.