Put the salmon, horseradish, lemon
zest and half the dill in a food processor,
season and blitz to a fine paste.
Shape the mixture into 4 burgers.
Heat the oil in a large frying pan and
cook the burgers for 4 mins on each
side until golden.
Meanwhile, mix the remaining dill
with the mayonnaise and spread a little
onto the base of each roll. Toss the
watercress in the lemon juice and
put a handful on each roll base. Top
with a burger and finish with slices
of cucumber and radish. Replace roll
tops to serve, if you like, or serve