To cook the asparagus, bring
Spread the salmon mousse on the potato cakes. (If you are using smoked salmon and sour cream, spread the sour cream on the potato cakes, then place the smoked salmon on top.) Place an asparagus spear on each cake and roll the cakes up around the asparagus. With a very sharp thin knife, cut each potato cake into 4 or 6 slices and arrange them on a serving plate cut side up. Add a little spoonful of salmon roe and a small sprig of dill to each canapé.