Wash the prawns, dab dry. Wash dill, dab dry, pluck little flags from the stems. Put some flags aside for garnishing. Chop the rest finely. Add the prawns and 1/3 of the dill to 125 ml sherry.
Leave to soak for about 10 minutes. Mix mayonnaise, sour cream, mustard, sugar, 2 tablespoons sherry and the remaining dill. Season to taste with salt and pepper. Wash the salad and dab dry. Put into the glasses.
Remove the prawns from the marinade and distribute in the glasses. Pour mustard sauce over them. Garnish with dill.