Place the salmon fillet on a flat surface and cut on the bias, against the grain, into 8 slices of equal weight. Place the slices between sheets of freezer paper and gently flatten the salmon with a meat pounder, mallet, or a heavy skillet into thin scallopini.
Melt a tablespoon of butter in a saucepan and add the shallots. Cook for a minute over medium heat. Do not brown shallots. Add the vermouth and wine. Simmer to reduce by half, then add fish stock. Cook for about five minutes, and strain the liquid, through a fine sieve while pressing all solids with the back of a ladle to extract their juices. Discard all solids. Add the cream to the liquid and simmer for 5 minutes.
Blend the remaining tablespoon of butter with the flour. Add bits by bits to the simmering sauce while stirring with a whip. Add the sorrel and cook until wilted. Season with salt and pepper to taste. Stir in the lemon juice. Keep sauce hot.
Using a non stick pan, cook the salmon pieces in the pan until golden on one side. Turn and cook on the other side for about a minute.
Spoon equal portions of the sauce in the center of hot plates. Place one piece of the cooked salmon in the center of the sauce. Serve immediately.