In a medium mixing bowl, combine the marinade ingredients.
Place brisket in a large non-reactive bowl or baking dish. Pour the marinade over the top. Cover completely and marinate brisket in the fridge for at least 14 hours, maximum 24. Halfway through marinating, flip the brisket so both sides can absorb the marinade.
After 14-24 hours, remove the brisket from the marinade and place in a large roaster. Reserve two cups of the marinade. Combine with the onion soup mix, tomato sauce, and ketchup. Slowly pour the gravy over the brisket. Top with sliced onions and bay leaves. Cover the roaster with aluminum foil.
Braise at 325 degrees for four hours. Halfway through, remove the brisket from the oven and spoon some of the gravy from the bottom of the pan over the brisket. Return to the oven to continue cooking.
Allow brisket to rest for 30 minutes before slicing. Slice brisket against the grain. Serve with reserved gravy.