If serving with potatoes and carrots, add during last hour of cooking.
If you want a thicker sauce, after brisket is done cooking, remove meat from pan. Place roasting pan over two burners over medium heat and boil vegetables until sauce is thickened (10-15 minutes). This is only if you want a thicker sauce and you don't want to let it reduce too much. If it reduces too much, you won't have enough sauce to cover brisket.
When serving, always cut brisket against the grain.