In a small saucepan, bring vinegar, ¼ cup water, sugar, salt, and coriander seeds to a boil together over medium-high heat. Cook for 2 minutes, stirring occasionally. Remove from heat, and pour over onion. Let cool for 30 minutes. Transfer mixture to an airtight container, and refrigerate until needed, up to a day.
Preheat oven to 350°. Line a rimmed baking sheet with parchment paper.
Using a serrated bread knife in a gentle sawing motion, cut each bread slice into a 4×2-inch rectangle. Cut rectangles in half, creating 2 (2-inch) squares. Cut each square in half diagonally, creating 2 triangles. Place bread triangles on prepared baking sheet. Brush with melted butter.
Bake until bread is crisp, 7 to 10 minutes. Let cool completely.
In a small bowl, stir together cream cheese, dill, and chives until well combined. Spoon mixture into a piping bag fitted with a ¼-inch-round piping tip (Ateco #802). Pipe approximately ½ teaspoon cream cheese mixture onto each toast. Top with 1 trout piece and 1 onion slice. (Reserve remaining onion for another use.)
Garnish with dill fronds, if desired. Serve immediately.
Notes
Make-Ahead Tip: Bread triangles can be prepared several hours in advance, cooled, and stored at room temperature in an airtight container until needed.
Recipe by TeaTime Magazine at https://teatimemagazine.com/smoked-trout-toasts-pickled-red-onion/