Mince the shallot and garlic, quarter the mushrooms and crush the walnuts.
Sprinkle pork lightly with salt and pepper. In a frying pan just large enough to hold all the meat in a single layer, heat butter over moderately high heat until it bubbles. Brown pork on both sides.
Add shallot and garlic around edges. Sprinkle with mushrooms and tarragon. Blend lemon juice into mustard, then mix in wine; pour over pork. Reduce heat, cover and simmer 15 minutes.
With a slotted spoon, remove pork to a warm serving dish; sprinkle with walnuts. Add cream to liquid in pan. Bring to a boil, stirring, and cook until sauce is reduced and slightly thickened. Add salt to taste. Pour over pork.