Skin, bone and flake the salmon and put in a large mixing bowl. Mix the cheeses and the sour cream together then combine with the salmon and add the gerkins. Dissolve the gelatine following the packet instructions and blend into the salmon mixture. Add the celery, onion, salt dill and lemon juice and mix well. Fold in the whipped cream until incorporated. Pour into wetted molds and refrigerate for at least 2 hours, or until set.
To make the sauce, peel and seed the cucumber and chop finely. Mix with all ingredients except the cream and leave for 30 minutes then strain off the liquid and mix with the cream. Unmold the mousse and serve with the sauce.