This cold-cured fish and popular appetizer needs to stand in the refrigerator for at least one day, so plan ahead. The resulting fresh, delicate taste will be worth the wait.
Rinse salmon; pat dry with paper towels. Trim excess fat and bones, and pull out pin bones using kitchen tweezers.
Combine fennel, sugar, salt, lemon zest, peppercorns, and caraway seeds. Line a rimmed baking sheet with plastic wrap.
Spread a quarter of the curing mixture onto plastic wrap. Place 1 salmon piece, skin side down, on top. Rub salmon with half the remaining curing mixture, and drizzle with aquavit. Top with remaining salmon, skin side up, and rub with remaining curing mixture.
Wrap salmon tightly in the plastic wrap. Set a baking sheet on top of salmon, and weigh down sheet with cans or bricks. Refrigerate, flipping salmon halfway through, for 1 to 2 days (the longer the fish cures, the denser the flesh will be).
Unwrap salmon; discard liquid, and wipe off remaining curing mixture. Thinly slice salmon on the bias with a long, thin knife. Spread sweet mustard sauce on bread. Top with gravlax, onion, and fennel.
Wiped gravlax can be refrigerated, wrapped in plastic, for up to 5 days.