How to Make It
Place cucumbers, vinegar, dill, sugar, ½ teaspoon salt, and ¼ teaspoon pepper in a medium bowl; toss to combine.
Stir together trout, crème fraîche, horseradish, 1 tablespoon lemon juice, and remaining ¼ teaspoon each salt and pepper until evenly combined. Taste and add remaining 1 tablespoon lemon juice, if desired.
Spread butter, if using, on warm toasts and top with white fish salad and cucumbers, drizzling with a little pickling liquid.