Mix salt, sugar, dill, and pepper and thickly coat the "meat" side of both fillets. Place fillets together, skin sides out, meat sides in, and bind together with a string or a rubber band. Place in a resealable bag and keep cool for 24 hours, draining the juice several times. Open the bag and remove the salt mixture from the fillets. Wipe them with a damp cloth and brush lightly with olive oil.
Serve with A-1 Sauce, spicy mustard, or horseradish--or a combination of all three.
Tuna is a little tougher than salmon; the fish usually prepared as gravlax, so if you have a choice, use salmon. If you use tuna, slice the fillets thinly.
Place in a clean resealable bag, squeeze out the air, seal, and store in the fridge or another cool location onboard.