1Trim all excess fat from brisket.
2Combine onion soup mix with mushrooms and their liquid.
3Place brisket in crockpot with fat side up, cutting to fit if necessary.
4Spread onion soup mixture over top of brisket, moistening well.
5Cover and cook on low for 10 to 14 hours.
6Remove brisket and cut across the grain into thin slices.
7Serve with meat juices poured over top of slices.