Cut off the crusts of the bread slices. Sauté the bread golden brown on both sides in a little butter. Place on paper towels. If the prawns are large, cut them into smaller pieces. Save four sprigs of dill for garnishes. Finely chop the rest of the dill and mix with the prawns, mayonnaise and mustard. Apportion the mixture on the slices of sautéd bread. Shape the whitefish roe like eggs and place on top of each toast. Garnish each with a sprig of dill and serve with a slice of lemon.