With an electric mixer beat the butter until smooth and soft, then add the almonds, dill, salt and pepper. Place the butter in the refrigerator to chill.
Peel the cucumber and cut into thin slices (easiest using a mandoline slicer).
Slice the demi-baguettes open and spread the butter on all baguette halves. Arrange half of the cucumber slices in a gill-like pattern on all the bottom halves. Then arrange the trout fillets on two baguettes, the salmon fillets on the other two baguettes.
Cover the fish with another layer of cucumber slices and close the sandwiches with the top halves of the demi-baguettes.