Remove the bones. Run the back of a chef's knife along the surface of the salmon to help reveal any bones. Use tweezers to pull out the bones, dipping the tweezers in water so the bones slip off. Pat the salmon dry with paper towels and set aside.
Make the cure. Mix the salt, dill, fennel seeds, coriander, peppercorns and brown sugar in a bowl.
Prepare the onions. Toss the onions and lime juice in a medium nonreactive bowl.
Cure the salmon. Spread half of the salt mixture on a large sheet of plastic wrap, then top with half of the onions. Place the salmon on top. Spread the remaining onions and salt mixture on the salmon, making sure to put a little extra around the sides so the fish is completely covered. Wrap the salmon tightly in the plastic wrap. Place in a baking dish to catch any liquid that might leak. Refrigerate 24 to 36 hours.
Rinse and dry. Carefully remove the plastic wrap and discard it (there will be a lot of liquid). Reserve the onions to serve with the salmon. Rinse the salmon under cold water and pat dry.
Slice the gravlax. Use a carving knife to cut the salmon in half lengthwise.
Trim off any remaining dark flesh from the skin side of each piece. Slice on the bias as thinly as possible, wiping your knife with a cold damp towel between slices. To store, wrap the gravlax in plastic wrap and refrigerate up to 5 days.