Thaw fish, if frozen. Measure thickness of fish. Cut into 4 serving-size pieces, if necessary. In a large skillet combine 1 cup water and 1/2 teaspoon salt.
For the bouquet garni place 1/2 cup of the watercress, the 2 tablespoons tarragon, lemon slices, peppercorns, and bay leaves in the center of a double-thick, 9-inch-square piece of 100-percent-cotton cheesecloth. Bring the corners of the cheesecloth together and tie with clean 100-percent cotton string. Place the bouquet garni in the skillet. Bring water to boiling; reduce heat. With a slotted spatula, gently lower the fish fillets into the water. Simmer, covered, for 4 to 6 minutes for each 1/2 inch of thickness or until fish just flakes when tested with a fork. Using a slotted spoon, carefully remove the fish fillets to a platter. Cover fish; keep it warm while preparing sauce.
For sauce, in a blender container or food processor bowl combine sour cream, remaining 1/2 cup watercress, the 1 teaspoon tarragon, 1/2 teaspoon ground pepper, and 1/4 teaspoon salt. Cover and blend or process until smooth. Stir in chives. Spoon sauce onto plates and top with fish. Serve with Herbed Beets. Garnish with additional watercress and tarragon, if desired. Makes 4 servings.
Cut green tops off beets; discard tops. Wash beets; do not peel. In a medium saucepan bring a small amount of lightly salted water to boiling. Add beets. Cook, covered, over medium heat about 20 minutes or until crisp-tender; drain. Let beets stand at room temperature about 10 minutes or until cool enough to handle. Holding beets under running water, slip skins off beets.
Place beets in a serving bowl. Add olive oil, vinegar, and salt. Toss to coat. Sprinkle with watercress and tarragon.