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.