Place 1/2 of the brisket in slow cooker. Top with onion.
Mix Seasoning Mix, cola, ketchup and brown sugar until blended. Pour 1/2 of the cola mixture over brisket. Top with remaining brisket. Pour remaining cola mixture over top. Cover.
Cook 8 hours on LOW. Remove brisket to serving platter.
Slice brisket across the grain into thin slices. Stir sauce before serving. Serve with brisket.