Brisket is a tough cut of meat and there’s simply no way around that. A sloooow, braising method is ideal and that said, this recipe will take time. But it’s worth it, promise.
Add the garlic, dark brown sugar, and cider vinegar. Season with kosher salt and pepper and cook until the onions are very soft and beginning to caramelize, about 5 mins more.
Stir in the tomato paste, mustard, soy sauce, and tomato sauce. Stir well. Add the 2 cups of the beef stock, stir, and return the brisket and any accumulated juices back to the pan.