For salmon in champagne sauce, smear a little butter over the base of the frying pan, then arrange the fillets in it. Now slowly pour the Champagne over the salmon, then bring it to a simmer over a medium heat. Spoon the Champagne over the top of the fillets before putting the lid on. Poach the salmon for about 8/10 minutes. The tip of a knife inserted into the thickest part will show if it is cooked when you just ease the flesh back.
While the salmon is poaching, melt the butter in a medium saucepan and cook the shallots in it over a gentle heat for 5/6 minutes, until softened and golden. When the salmon is cooked, carefully lift the fillets on to a warmed dish, cover them with foil and keep warm. Next, add the flour to the buttery shallot juices, stir it in and cook for 1/2 minutes.
Gradually add the salmon poaching liquid to the pan, a little at a time, then blend in the double cream, whisking until the sauce is smooth. Let it come to a simmer and cook for 1/2 minutes, taste and add some seasoning. Serve the salmon on warmed plates with a little of the sauce spooned over and a teaspoon of keta and a sprig of dill on top, and hand the rest of the sauce round in a warmed jug.