Trim and finely chop the spring onions, and pick and finely chop the parsley. Beat the egg.
Combine the crabmeat, potatoes, spring onion, parsley, white pepper, cayenne and egg in a bowl with a little sea salt.
Refrigerate for 30 minutes, then shape into 6cm cakes.
Dust with flour and shallow-fry in oil over a medium heat for about 5 minutes each side or until golden-brown.
Serve with pinches of watercress and a dollop of tartare sauce.