You can't get much more basicthan poaching; cooking thesalmon in a packet in saltedwater with lemon and peppercorns gives you moist, healthyresults.
Bring 6 quarts water, 6 tablespoons lemon juice, lemon halves, 1 tablespoon salt, and the peppercorns to a boil in a large, wide pot. Reduce heat; let simmer.
Season two salmon steaks with salt. Place one-quarter each of the fennel and onion and 2 sprigs each dill and parsley on a long sheet of parchment; top with salted steaks, one-quarter each fennel and onion, and 2 sprigs each dill and parsley. Fold parchment to make a packet. Repeat process to make another packet.
With kitchen twine, tie one packet to a wire rack; place in pot. Cover; poach until cooked through, about 10 minutes.
Use tongs and a spatula to transfer rack to a rimmed baking sheet. Carefully open packet. Remove skin and any brown fat; discard. Transfer fish to a platter, discarding vegetables; cover. Cook other packet. Reserve 1 tablespoon cooking liquid.
Whisk mayonnaise, sour cream, 2 tablespoons lemon juice, and cooking liquid in a bowl. Stir in chopped dill and chives; season with salt and pepper. Garnish salmon with dill sprigs; serve with sauce.
Make the fish to eattonight, or cook it today to enjoycold tomorrow. Add a saladand a vegetable, and you have anelegant, uncomplicated meal.