Remove head from salmon, if desired. Place salmon on double thickness of wide foil, making sure foil is three to four inches longer than fish at each end. Sprinkle fish inside and out with salt and pepper to taste. Combine mushrooms, onion, parsley, cheese, lemon peel, and juice. Spoon mixture into fish cavity. Pour butter over fish and top with lemon slices. Cover with another thickness of foil and carefully seal all sides completely. Place on grill four to six inches from glowing coals. Turn after 30 minutes and cook 20 to 30 minutes longer. If cooking on smoke-type grill, open foil during last ten minutes and close grill cover so smoke flavor penetrates fish. Serve with choice of sauces.