In a medium bowl, mix together the sour cream, lemon zest, and lemon juice until well combined.

In another medium bowl, combine the hard-boiled eggs, dill, red onion, and capers (if using).

Stir half of the sour cream mixture into the egg mixture, ensuring it is evenly distributed.

In a separate bowl, combine the shredded smoked trout, celery, parsley, caraway seeds (if using), and pepper.

Add the remaining sour cream mixture to the trout mixture and mix until everything is well incorporated.

To assemble the sandwiches, divide the egg mixture evenly among 4 slices of bread.

Top each with another slice of bread.

Spread the trout mixture evenly over the bread.

Top with the remaining bread slices.

Carefully cut each assembled sandwich in half.

Serve immediately or chill for later.