Use a stainless steel or enameled skillet large enough to hold salmon in one layer. Butter the pan well and sprinkle shallots over the bottom. Add salmon on top of shallots. Pour wine and clam juice over salmon. Bring to a boil over moderate heat. Poach the salmon, covered, for 20 minutes, or until it just flakes when tested with a fork.
Transfer it with a slotted spatula to a plate, and skin it while it is still warm. Cover the salmon and chill it for at least one hour. Reduce the cooking liquid over high heat to about 1/4 cup and let it cool completely.
