How to Make It
In a medium nonreactive saucepan, combine the clam juice, wine, shallot, parsley stems, thyme sprigs, bay leaf and 1 cup of water. Boil over high heat until reduced to 2 cups, about 25 minutes. Strain the reduction into a bowl.
Wipe out the pan. Add 1 tablespoon of the butter and melt over low heat. Stir in the flour to make a roux and cook, stirring, for 2 minutes. Remove from the heat and let the roux cool for 5 minutes.
Set the saucepan over moderate heat and gradually whisk the reduction into the roux to make a smooth sauce. Bring to a boil, whisking constantly. Reduce the heat to very low and simmer the sauce gently, whisking often, until no floury taste remains, about 10 minutes. Stir in the cream and simmer for 5 minutes longer. Remove from the heat.
Bring the Court Bouilon to a simmer. Add the salmon fillets and adjust the heat so that the liquid is barely simmering. Poach the salmon until just cooked through, about 6 minutes, depending on the thickness of the fish.
Meanwhile, bring the sauce to a simmer over moderate heat. remove the pan from the heat and whisk in the remaining 1/2 tablespoon of butter. Stir in the sorrel to wilt it and season the sauce with salt, white pepper and lemon juice, if using. If the sauce seems thick, thin it with a little of the Court Bouilon; the sauce should have the consistency of light cream.
Using a slotted spoon or spatula, remove the salmon from the poaching liquid and drain briefly on paper towels. Set the salmon on warmed plates and spoon the sorrel sauce on top. Serve immediately.