Preheat oven to 325 degrees F.

Combine salt, pepper, cumin, chili powder, oregano, paprika, garlic and oil in a small bowl. Rub seasoning over the side of the beef without the fat cap and let sit for 20 minutes. Place the brisket, fat side up, in a roasting pan. Add the beef stock and beef, cover and braise in the oven for 3 ½ to 4 hours or until brisket can easily be shredded with a fork. Remove from oven. Skim fat from cooking liquid and reserve 1 cup.

Prepare the onion jam. Heat oil in a nonstick saute pan over medium heat. Add onions and cook until caramelized about 20 minutes. Add a little bit of water if the pan becomes too dry. Add flour and stir to coat the onions. Add balsamic vinegar, brown sugar, thyme, and reserved cooking liquid. Bring to a boil, reduce heat to low and simmer for 10 minutes. Season with salt and pepper. Serve with brisket.