Like salmon, the usual choice for this Swedish delicacy, cured halibut is delicious. For food safety, it's best to use thawed frozen-at-sea (FAS) halibut. Depending on the size of the fillets, you may use two to five pieces. Don't use one large fillet, as the salt cure will not seep evenly throughout the fish. Slice the gravlax into paper-thin strips with a thin, sharp knife, angling the blade for wider slices. Serve as an appetizer with rye or pumpernickel toasts and cream cheese. Garnish with dill sprigs.