Place the peppercorns, dill and four lemon slices(reserve the remaining slices for the garnish) in a large saute pan and pour in the boiling water. Add the salmon, cover with a tight fitting lid and cook for 5 minutes. Turn off the heat and allow salmon to cool completely in the water-this should cook it perfectly. Remove from the pan then transfer to a plate, cover and chill until ready to serve.
To make the sauce, place the cucumber and dill in a food processor and whizz until smooth. Add the creme fraiche and some salt and pepper and pulse to make a smooth, pourable sauce.
To serve, place a salmon fillet on 4 plates and pour over the sauce. Garnish each with a slice of lemon and a sprig of dill.