Place 1/2 of the brisket in slow cooker. Top with mushrooms and onion.
Mix Seasoning Mix, flour, water and Worcestershire sauce until well blended. Pour 1/2 of the mixture over meat and vegetables. Top with remaining brisket. Pour remaining seasoning mixture over top. Cover.
Cook 8 hours on LOW or 4 hours on HIGH. Remove brisket and vegetables to serving platter.
Slice brisket across the grain into thin slices. Skim fat from sauce. Serve over brisket and vegetables.