Add garlic into a food processor, pulse until finely chopped. Add the brown sugar, mustard, oil, black pepper, cumin, paprika, cayenne, ¼ cup salt and process until a smooth and slightly runny paste is formed, set aside. Remove brisket from packaging and dry with a paper towel and place on a baking tray or large dish. Rub paste all over brisket being sure that its well covered. Cover with plastic wrap and let chill for at least 24 hours.
Before cooking, let meat sit out until room temperature, about 1 hour. Preheat oven to 325°. Place onions in a large roasting pan and place brisket, fat side up, on top. Pour in the beer; cover with foil. Braise until meat is very tender, approximately 5–6 hours. Remove foil and heat broiler. Broil brisket until top is browned and crisp, approximately 5–10 minutes.
Remove from the oven and let the brisket rest for 10 -15 minutes. Remove from pan and slice. If needed, you can moisten the meat with the braising liquid and season with salt before serving.
Recipe by jessica burns at http://jessicaburns.com/2015/03/beer-braised-brisket/