In a medium bowl, combine brown sugar, smoked paprika, garlic powder, onion powder, black pepper, salt, and cumin to form a spice rub. Rub this mixture generously over the entire surface of the brisket, ensuring it's well-coated. Wrap the seasoned brisket tightly in plastic wrap and refrigerate for at least 4 hours, preferably overnight.

Preheat your oven to 300°F (150°C). In a large roasting pan, create a bed for the brisket by layering sliced onions and crushed garlic at the bottom. Place the brisket, fat side up, on top of the onions.

In a separate bowl, combine beef broth, cola or Dr. Pepper, Worcestershire sauce, apple cider vinegar, and tomato paste to create a flavorful cooking liquid. Pour this around the brisket in the roasting pan, ensuring the liquid does not wash off the seasoning.

Cover the roasting pan tightly with aluminum foil and bake for about 4 hours, or until the brisket is tender. Check at the 3-hour mark to ensure the liquid level is sufficient.

Once cooked, remove the brisket from the oven and let it rest for about 20 minutes. Skim off excess fat from the cooking liquid.

In a skillet, melt butter over medium-low heat. Add thinly sliced onions with a sprinkle of brown sugar and cook slowly for about 20-30 minutes until softened and golden brown.

Shred the rested brisket with two forks and mix with the reduced cooking liquid and caramelized onions.