Carefully flake poached halibut. Peel, core and slice apples thinly. Combine butter and cider in a frying pan and sauté apple slices until soft, stirring constantly. Force apples and pan juices through a fine sieve and add sour cream, grated horseradish, prepared mustard and lemon juice.
Beat mixture until light and foamy, adding a little more cider if necessary, and toss flaked fish ge