Line the bottom of the slow cooker with the onions. Then top with brisket and season with salt and pepper.
In a bowl, add together all the sauce ingredients and mix well. Pour sauce over brisket and cook on low for 6-8 hours.
Towards the end of the cooking, use a spoon to skim off any excess fat from the sauce.
To serve, place on platter and drizzle sauce on top.