First of all, smear a little butter over the base of a large frying pan, then arrange the fillets in it.
Now slowly pour the Champagne over the salmon (it will foam quite a lot, but not to worry), then bring it to a simmer over a medium heat. Now spoon the Champagne over the top of the fillets before putting the lid on.
Gently poach the salmon, covered, 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 but not browned.
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 more.
Now gradually add the salmon poaching liquid to the pan, a little at a time, then blend in the cream, whisking until the sauce is smooth. Let it come to a simmer and cook for 1-2 minutes, then season with salt & pepper to taste.
Serve the salmon fillets on warmed plates with a little of the sauce spooned over and a teaspoon of keta (optional) and a sprig of dill on top, along with fresh lemon wedges and hand the rest of the sauce round in a warmed jug.
Very nice accompanied with fresh, shelled peas, or a green salad with plenty of cucumber and a lemony dressing.