Trim all excess fat from brisket. Combine onion soup mix with mushrooms and their liquid. Place brisket in crockpot with fat side up, cutting to fit if necessary. Spread onion soup mixture over top of brisket, moistening well. Cover and cook on Low for 10 to 14 hours.
Remove brisket and cut across the grain into thin slices. Serve with meat juices poured over top of slices.
Serves 8 to 10.