This is a rich and satisfying smoked fish appetizer for parties or weddings.
Preheat oven to 300 degrees. Arrange bread on two baking sheets; place in oven to dry out and toast slightly, 5 to 7 minutes per side. Transfer to a wire rack to cool.
In bowl of food processor, combine trout, mayonnaise, shallot, vinegar, horseradish, and Worcestershire until smooth. Whip cream to soft peaks; fold into trout mixture. Cover; chill 1 hour.
Slice tomatoes in half; squeeze out seeds. Use flower-shaped aspic cutter to cut out tomatoes; cover and refrigerate.
Using a 2 1/4-inch-square cookie cutter or serrated knife, cut bread into squares. Cut each in half into two triangles. Place mousse in pastry bag fitted with Ateco #863 star tip; pipe mound in center. Top with tomato flowers and chervil. Serve.