Peel the skin off the trout and flake the flesh into a medium bowl. Add the pickle, celery, onion, scallion and dill and toss well. Drizzle the lemon juice and oil on top, season with the pepper and toss well. Refrigerate until ready to serve (up to 2 hours).
In a small bowl, stir together the horseradish and creme fraiche. Add the celery leaves to the smoked fish mixture. Spread bagel halves with cream cheese, spoon on some of the smoked fish and drizzle with the horseradish cream.