Add the garlic, dark brown sugar, and cider vinegar. Season with kosher salt and pepper and cook until the onions are very soft and beginning to caramelize, about 5 mins more.
This recipe is ideal for a 4 – 4 1/2 lb brisket. If you have a larger brisket the cooking time may be longer and you will probably need to use a roasting pan instead of a dutch oven.