Fry the onion in oil until soft and golden brown and set aside to cool.
Stir the pilchards, potato, onion, egg and coriander together with a fork.
Form the mixture into balls about the size of an egg and pat into the shape of a patty. Use water to stop the mixture from sticking to your hands. If it's too wet you can add a tsp of flour at a time to bind the fishcake mixture.
Dust with flour and shallow fry until golden brown, drizzle patty with pilchard sauce.
Assemble the burger by spreading mayonnaise on the bottom half of the bun.
Add lettuce tomato and onion then top with a patty.
Drizzle with mayonnaise and top with the remaining half of the bun.
Serve with salad and drizzle with remaining pilchards sauce.