Toss onions with seasoning or salt and pepper, yuzu, Ponzu or lime juice; let stand at least 30 minutes.
Pulse tuna in a food processor to finely chop then gently combine with garlic, ginger, scallions, pepper and Ponzu or Tamari. Form into patties and coat with oil.
Assemble the toppings and heat a griddle or nonstick skillet.
Combine sauce ingredients.
Cook burgers 4-5 minutes, turning once for pink centers and 7-8 minutes to cook through.
Serve burgers on toasted buns topped with toasted Nori, pickled onions, pickled ginger and special sauce.