Fold 36-inch length of 18-inch foil into square; seal sides to form bag.
Put in rice, mushrooms, water, onion, salt, and sauce; mix carefully.
Dot with butter.
Fold over top edges of foil to seal bag tightly.
Place on barbecue over hot coals about 1/2 hour.
Before serving, open foil; add a little extra butter.
Fluff up rice with fork.