1. In a mixing bowl, combine together the cheese and sour cream; beat well to mix.
2. In a separate bowl, put the aspic crystals and pour over the boiling water; stir to dissolve well.
3. Stir in the aspic jelly and leave in a cool, dry place until ready for setting.
4. Beat the egg whites until stiff, then fold into the mixture and pour into a dampened 450 g/1 lb loaf tin.
5. Chill in the refrigerator until the mousse has set.
6. To freeze: open (flash) freeze until firm, then turn the mousse out of the tin and wrap in foil.
7. Seal and return to the freezer; label the bag as it makes it easier to distinguish.
8. To thaw : remove wrappings and arrange the mousse on a serving platter.
9. Thaw in the refrigerator overnight.
10. Garnish the mousse with sour cream and serve cold.