Combine smoked trout, cream cheese, mayonnaise, horseradish, minced red onion, celery and parsley in a large bowl. Stir with a wooden spoon, until smooth, but there should still be chunks of smoked salmon in the mixture. Add lemon juice and salt & pepper to taste. Serve on top of squares of pumpernickel bread and on slices(1/3-in (1 cm) thick) of seedless cucumber, that have a bit of the centre scooped out to create a small impression to hold the smoked trout spread.