Heat 3 inches of oil in a medium Dutch oven until the oil reaches 365 degrees F.
Whisk together the mayonnaise, vinegar, sugar, togarashi and wasabi in a small bowl. Top with the pickled ginger. Refrigerate until ready to serve.
Peel the onions and cut them crosswise into 1/2-inch-thick slices. Separate the slices in rings. Set up a dredging station with two shallow dishes. Pour the buttermilk into the first dish. In the second dish, whisk together the panko, flour, sesame seeds, cayenne, garlic powder, onion powder and salt. Dip each onion ring in the buttermilk and then in the seasoned panko mixture. Set on a baking sheet in a single layer until you have dredged all the onion rings.
Working in batches, gently add the breaded onion rings to the hot oil and fry until golden brown and crispy, about 4 minutes, flipping them over halfway through. Remove to a paper-towel-lined baking sheet and season with togarashi.
Serve the onions rings with the wasabi dipping sauce.
Togarashi powder is available in many Asian markets.