Sprinkle garlic salt and place sliced onion on brisket and bake at 3500 in covered pan (wrap meat in foil) for 3 1/2 to 4 hours, or until fork-tender. Remove foil-wrapped meat and refrigerate overnight. Add mushroom gravy to pan drippings. Add remaining ingredients. Refrigerate gravy. When ready to use, slice cold beef. Add the gravy to the beef and reheat in foil-covered pan. Can be kept covered on oven warm setting or warming tray for a long time.