The classic dressing for cooked salmon is aspic, finely sliced cucumber, lemon and herbs - such as dill and fennel. Aspic jelly is easy to find in the shops and is great for keeping fine slices of cucumber or lemon in place. Or you could use 300ml (half a pint) of thick cold mayonnaise to pipe rosettes over the fish. (Do this as near to the serving time as possible.) Place peeled prawns into the mayonnaise rosettes and garnish with sliced lemon and cucumber or salad leaves, fresh watercress or sprigs of dill.