Gut and wipe clean the salmon then take two large fillets from each side, as close to the backbone as possible (use the carcass for stock or soup).
Roughly chop the dill and mix thoroughly in a bowl with the salt, sugar and pepper. Lay out a sheet of foil or cling film about four times the width of a salmon fillet. Spread a quarter of the pickling mix over a fillet-size area on one side of the foil with a good 15cm/6in margin for folding over.
Place one fillet, skin side down, on top of the pickle mixture and cover with slightly more than half of what is left. Place the second fillet on top, skin side up, to make a sandwich. Scatter the remaining pickle mixture over the skin.
Wrap up the parcel tightly, tucking the ends and edges in underneath the fish.
Put the package on the tray and place a similar size tray, or a plank of wood on the top. Weight it down, with a brick or two or anything else handy (the contents of the fridge?).Turn the package daily for at least five days, and no more than eight. Do not discard the pickling liquid that oozes from the package unless it threatens to spill over the side of the tray.
At least one hour before you wish to serve the fish, combine all the ingredients for the dressing in a jar and shake well together. Leave to stand and shake again to emulsify before serving.
To serve, unwrap the gravadlax and wipe off any excess pickling liquid. There is no need to scrape off the dill pieces though, as some recipes suggest. Slice fairly (but not too) thinly then serve with the sauce and buttered brown bread. (Unused gravadlax can be re-wrapped in clean cling film or foil and kept in the fridge for up to five days.)