Place each fillet on a piece of aluminum foil that has been sprayed with non-stick vegetable spray. Top fillets evenly with onion slices, diced butter, lemon slices and brown sugar. Fold over aluminum foil tightly to make packets; make several holes in top of packets with a fork to allow steam to escape. Arrange packets on an ungreased baking sheet. Bake at 375 degrees for 15 to 20 minutes.
This was very simple to assemble and in general was very good. My salmon fillets were about 1 inch think and I cooked them for 17 minutes, but think they would have been better cooked for a few minutes less as the salmon was a bit dry. The combination of onion, lemon, butter and brown sugar made for an interesting flavor combination. I placed the cooked salmon on our dinner plates without adding the remaining liquid in the packet, but I think it would have been better to put the liquid on top of the salmon or the rice that we served with the salmon.