Peel the potatoes while still hot. Push the hot pieces of potato through a sieve into a bowl and leave to cool a little. Beat the egg.
Mix the flour, cheese, egg, salt and nutmeg with the mashed potato, adding a little flour if necessary. The mixture should still be moist but no longer stick to your hands.
On a lightly floured surface, shape the gnocchi mixture into 4 rolls (each approx. 1.25 cm in diameter).
Cut the rolls into pieces approx. 2 cm long.
Shape into balls and roll over the prongs of a fork to create a pattern. Poach the gnocchi in 4 batches in a pan of lightly salted water for approx. 5 mins. per batch. Remove with a slotted spoon, drain, cover and keep warm.
Finely chop the shallot. Crush the peppercorns. Bring the wine to the boil with the shallot, tarragon and peppercorns and reduce for approx. 5 mins. to half the amount. Strain, return to the pan.
Whisk the egg yolk and cream, add to the sauce, bring to the boil while stirring constantly, reduce the heat. Continue stirring until the sauce thickens slightly.
Cut the salmon into strips approx. 2 cm thick, add to the sauce and heat gently, season. Serve the salmon sauce with the gnocchi.