To Prep
Fill enough water to cover egg in a small sauce pan and bring it to a boil. Place egg in and let it simmer under medium heat for 9 mins. Let boiled egg sit for 10 mins, remove from saucepan and remove shell and let it cool.
Mash egg into fine crumbs. Add in all the tartar sauce ingredients and mix well. Set aside.
Heat up frying pan with oil and sauté onion until soften. Remove and set aside to cool.
Smash prawns with knife and give them a rough chop.
Place chopped prawns with sautéed onion, potato flour, salt, pepper and wasabi in a mixing bowl and mix them all until well incorporated.
Divide into 2 equal portions and flatten them into square patties.
To Make
Set up coating station of flour, beaten egg and panko. Coat prawn patties with flour then beaten egg follow by panko.
Heat up oil in a frying pan to 180°C and fry prawn patties one at a time until both sides turn golden brown. Drain oil and let them cool on cooling rack.
Alternatively, bake coated prawn patties sprayed with some oil in the oven at 180°C for 25 mins or 15 mins in the air fryer. Turn them over mid-way.
Place one prawn patty on a slice of bread, top with shredded cabbage and tartar sauce and cover with another slice of bread.