Make small slits along the bottom side of the shrimp to prevent curling.
In a bowl, combine onion and hanpen and mix until well combined.
Divide mixture into six even portions and place each portion on plastic wrap.
Place one shrimp on top of mixture and wrap it up so that only the tail of the shrimp is visible.
Take off the plastic wrap and coat each piece with flour, egg wash and finally the panko.
Deep-fry in hot oil until crispy and golden brown.
Serve with tonkatsu sauce.