Bring a large saucepan of salted water to the boil, add the potatoes and cook for 15-20 minutes.
Drain well, then mash with a potato masher or fork until smooth.
Put the fish in a frying pan and just cover with water. Bring to the boil over a medium heat,
then reduce the heat to low, cover and simmer gently for 5 minutes until cooked. Remove with a
slotted spoon and drain on a plate. When cool enough to handle, flake the fish roughly into
good-sized pieces, removing and discarding any bones.
Mix the mashed potatoes with the fish, tarragon, lemon rind and cream in a bowl. Season well
with salt and pepper and shape into 4 round cakes or 8 smaller ones.
Put the flour, egg and breadcrumbs in separate bowls. Dust the fish cakes with flour, dip into
the beaten egg, then coat thoroughly in the breadcrumbs. Put on a baking sheet, cover and chill in
the refrigerator for at least 30 minutes.
Heat the oil in the frying pan over a medium heat, add the fish cakes and cook for 5 minutes on
each side, turning them with a spatula. Serve hot, garnished with lemon wedges, with a watercress
salad to accompany.