Melt the butter. Sauté the shallot. Add the peppercorns and tarragon. Pour in the wine and bring to the boil. Reduce the heat, reduce the liquid to approx. half the amount for approx. 10 mins., uncovered. Remove the tarragon. Shortly before serving, add the mustard, pour in the cream and then bring to the boil while stirring. Blend with a handheld blender until frothy, season.
Cook the tagliatelle in boiling salted water until just al dente.. Add the leek approx. 3 mins. before the cooking time is up, drain and mix with a little butter.
Heat the clarified butter in a non-stick frying pan. Fry the salmon fillets for approx. 3 mins., turn the fillets and cook for a further 2 mins. Season the salmon fillets.