Whip the cooled cream until stiff and mix with the washed, finely chopped nettle leaves and horseradish. Seasoned with salt and pepper, pour the mixture into a small baking dish and set aside to cool.
When firm, cut out balls with a spoon – warmed in a hot water bath – and place them on the smoked salmon. The mousse can be used just as well as a spread for rolls, bread or baguette.