2½ to 3 lbs. salmon fillets, skin on, all bones removed
1½ T kosher salt (more if needed), and 2¼ t brown sugar
4 T cognac, plus more if needed
1 c dill sprigs, packed
Trim the salmon fillet, cutting away any thin uneven edges and the thin end of the tail (which can be reserved for something else). Make sure all the pin bones are removed–run your fingers up the fillet; if you feel any bones, remove them with a tweezer or a needle-nosed plier.
Cut the fillet in half crosswise so that you have two pieces of the same length and roughly the same width. Mix the salt and sugar together. Sprinkle half the mixture over each fillet and rub it in with your fingers.
Place one fillet in a glass (or other non-reactive) baking dish big enough to hold it. Drizzle about two tablespoons of cognac over each half, rubbing it in with your fingers. Spread the dill over the salmon half in the baking dish.
Lay the other half fillet on top (skin side up). Align the two halves. Cover closely with a sheet of plastic wrap. Place a board or pan on top of the fillets. Make sure it is resting on the fish and not on the sides of the baking dish. Weight the top with something heavy (a large can of tomatoes for example). Place in refrigerator.
After one day of curing, remove the weight and board and turn fillets over (so the top fillet is now on the bottom) and baste with the liquid that has accumulated in the dish. Replace weights and board and return to frig.
On the second day, turn and baste again and slice off a tiny piece to taste. Add a little more salt and cognac on the fish. Return to the fridge.
Cure for a third day, turn and baste again. On the fourth day, you can serve the gravlax.
To serve, clean the dill away and wipe the fish dry with paper towels. Use a long thin-bladed slicing knife (sharpened) and start slicing a few inches from the narrow end of the fillet. Cut with a back and forth sawing motion toward the narrow end to remove a thin slice of fish. Start each succeeding slice a bit farther in from the narrow end; always cut at a flat angle to keep the slices as long and thin as possible. Garnish with new, fresh dill sprigs and lemon slices.