1. Puree all smoked-salmon filling ingredients in food processor until smooth.Make Watercress Filling::
2. Process all watercress filling ingredients in food processor until well combined.
3. Lightly spread one side of all bread slices with butter. Spread 1 generous tablespoon of the salmon filling on each of 10 slices whole wheat bread. Spread 1 tablespoon of the watercress filling on each of 10 slices white bread.Stack two salmon slices with two watercress slices, alternating salmon and watercress slices; top with 1 whole wheat slice, buttered side down. Repeat, making 5 sandwich stacks.
4. Wrap sandwiches in wax paper and arrange on cookie sheet. Place second cookie sheet on top to weight lightly. Refrigerate at least 1 hour or up to 24 hours. To serve, unwrap sandwiches, trim crusts and slice into 1/2-inch-wide strips. Arrange on trays. Makes 30 sandwiches.