Combine the salt, sugar and pepper.
Divide half the salt mixture between the two fillets, rubbing into cut sides.
Turn fish and use the remaining salt to rub on the skin sides.
Place one fillet skin side down in a Ziploc bag.
Top with half the dill and all the onion.
Top this with the remaining dill and the other fillet, cut side down.
Seal the bag, pressing out the air.
Place bag in a bowl. Top with a small plate and weight it down with a couple of 1-lb cans.
Refrigerate for 48 hours, turning once or twice a day.
When ready to serve, discard dill and onion and pour off the accumulated moisture (do not drain this off before the 48-hour period is over).
Slice thinly on an angle.