In a small skillet, melt the butter over high heat. Add the mushrooms and saute until they give off some liquid, about 5 minutes. Remove from the heat and season to taste with salt and pepper.
Pour the wine into a saucepan large enough to hold the salmon and bring to a simmer. Slip in the salmon, cover pan, and poach gently over low heat until the salmon tests done (it should appear opaque when pierced with the point of a knife), 8 to 10 minutes. Remove the salmon from the poaching liquid with a slotted skimmer and set aside on a warmed plate. Keep warm.
Add cream and 1 tablespoon tarragon to the poaching liquid and reduce over high heat to a syrupy sauce. Add mushrooms and warm through. Spoon sauce over salmon and garnish with more tarragon and fresh lemon.