Grease a 6-cup fish mold with butter or mayonnaise.
Soften the gelatin in 1/4 cup cold water. Add 1/2 cup boiling water and stir well, until the gelatin has dissolved. Add 1/2 cup mayonnaise, 1 tablespoon lemon juice, lime juice, onion, hot sauce, paprika, and 1 teaspoon salt and mix well. Fold in the salmon and capers. Add the whipped cream and continue folding until everything is well combined. Pour the mixture into the prepared mold. Cover with plastic wrap and chill in the refrigerator for 8 hours or overnight.
To make the dill sauce, combine cucumber, sour cream, 1 cup mayonnaise, 1 tablespoon lemon juice, garlic, 1 teaspoon salt and dill in a medium bowl. Cover with plastic wrap and chill for at least 1 hour.
When ready to serve, un-mold the mousse onto a large plate. Take the lemon slices and create a tail on the back of the fish. Surround the mousse with parsley.
Serve the dill sauce in a glass bowl next to the salmon mousse.