1 Remove any pin bones from the salmon: Place the salmon flesh side up on a cutting board. With your fingertips, feel along the flesh to see if there are any fine pin bones. Use tweezers, a strawberry huller, or another pincher to gently pull them out.
2 Mix the curing mixture: In a bowl, combine the salt, brown and granulated sugar, and fennel seeds. Mix well.
3 Sprinkle half salmon with half curing mixture: Sprinkle some of the curing mixture down the middle of a 12-inch glass or ceramic baking dish and sprinkle some of the dill on top. Set one of the pieces of salmon, skin side down, on top of the salt mixture. Sprinkle half the remaining salt mixture and half of the remaining dill over the flesh of the fish. Press lightly so the mixture sticks.
4 Set the other piece of salmon, skin side up, on top of the first piece. Lay the second piece so that the thicker part of the top piece sits on the thinner part of the bottom piece, forming flat, even rectangle. Sprinkle the remaining salt mixture and dill over the top.
5 Cover and press the salmon: Cover the whole dish with plastic wrap. Set another, smaller baking dish on top of the salmon and weight it down with two heavy cans, a garden brick, or any other heavy weight.
6 Refrigerate and cure for 2 days. Flip the fish after 24 hours, so that the top piece is now on the bottom. This helps the fish cure evenly.
7 Rinse the curing mixture off the gravlax: When ready to serve, remove the fish from the baking dish. Some liquid will have collected on the bottom; this is normal. Rinse the fish well with cold water, rubbing off the curing mixture with your fingers. Dry the fish well with paper towels.
8 Slice the gravlax: Set one of the pieces of fish, skin side down, on a cutting board. Using a very sharp knife, slice the fish as thinly as you possibly can while holding the blade at an extreme diagonal. Do the same with the other piece of fish.
9 Serve the gravlax: Arrange the slices overlapping on a platter, curling them as you set them down. Sprinkle with red onion, capers, and remaining dill, and garnish with lemon. Leftovers will keep for about a week.