Melt 3 tablespoons butter in heavy large skillet over medium heat. Add shallots and saute 2 minutes. Increase heat to medium-high. Add mushrooms; saute until beginning to brown, about 8 minutes. Add clam juice and wine; boil until liquids are syrupy and almost evaporated, about 20 minutes. (Can be made 6 hours ahead. Cover and chill. Rewarm over medium heat, stirring frequently.) Add cream to mushrooms; boil until thickened, about 1 minute. Mix in chopped tarragon. Season with salt and pepper. Preheat broiler. Arrange salmon skin side down on broiler pan. Brush with lemon juice, then butter. Sprinkle with salt and pepper. Broil until just cooked through, without turning, about 6 minutes. Transfer to plates. Spoon mushrooms over. Garnish with tarragon. Serves 6. Bon Appetit October 1993