Cut the potatoes into long, triangular finger-length pieces. Place in a pan with the dill (keeping a few sprigs to garnish) and simmer gently in salted water.
Preheat the oven to 200C/400F/Gas6. Clarify 25g/1oz of the butter by melting it gently in a small pan then leaving it for 4-5 minutes until the solids fall to the bottom. Pour the clear butter off into the frying pan. Season the salmon steaks with salt and pepper. Heat the clarified butter, add the salmon and brown on both sides. Remove the pan from the heat and leave for 30 seconds. Pour over the wine, place the pan in the oven and cook for about 5 minutes.
Remove the salmon steaks from the pan and keep warm while you make the sauce. Pour the stock into the pan, bring to the boil and add the remaining butter. Boil rapidly to reduce the volume by half then add the parsley.
Arrange the salmon, watercress and the potatoes on four warmed plates. Mix together the olive oil, wine vinegar and ½ tsp of salt and spoon over the watercress. Pour the sauce over the fish and serve.
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