Cover a large cookie sheet (with sides) with foil. Lay one or two salmon fillets, skin side down, on foil. Cover fillet with 1/4 to 1/2 inch brown sugar. Lay onion slices on top of the brown sugar. Squeeze lemon juice over this. Refrigerate for at least 1-2 hours, spooning melted brown sugar and lemon juice back over the fillets every 1/2 hour.
25 minutes before eating, lay strips of bacon across the fillets. Broil on the lowest rack of the oven for 25 minutes. The bacon will turn dark.