Place 1/2 of the brisket and onion wedges in slow cooker.
Mix Seasoning Mix, ketchup, brown sugar and vinegar until blended. Pour 1/2 of the mixture over brisket. Top with remaining brisket and seasoning mixture. Cover.
Cook 8 hours on LOW or 4 hours on HIGH.
Remove brisket to serving platter. Slice thinly across the grain. Stir sauce before serving. Serve with brisket.