Cut fish filets into 2" pieces and place in large skillet with wine and salt. Poach gently until thoroughly done, about 15 minutes.
Drain fish well, discarding wine, and place in food processor with onions, capers, and seasonings. Remove to bowl and blend in mayonnaise and horseradish, adding a bit at a time until desired consistency is reached. Should not be too thin.
Chill overnight. Sprinkle with extra capers, and serve with crackers or crostini.