Mix the rub. In a small bowl, combine brown sugar, paprika, salt, pepper, cumin, and thyme. Pat the brisket dry and rub the mixture all over the meat.

Sear the brisket. Heat olive oil in a large skillet over medium-high heat. Sear the brisket 3–4 minutes per side until well browned. Transfer to the slow cooker.

Soften aromatics. In the same skillet, add the sliced onion and a pinch of salt. Cook 3–4 minutes until lightly browned. Stir in the garlic for 30 seconds. Transfer to the slow cooker, spreading the onions under and around the brisket.

Make the cooking liquid. Whisk together beef broth, Worcestershire, tomato paste, vinegar, and soy sauce (if using). Pour over the brisket. Add the bay leaf.

Cook low and slow. Cover and cook on Low for 8–10 hours or High for 5–6 hours, until the brisket is very tender and easily pulls with a fork.

Rest and slice. Carefully transfer the brisket to a cutting board. Tent with foil and rest 10–15 minutes. Slice against the grain into thick slices, or shred if you prefer.

Thicken the sauce (optional). Skim excess fat from the slow cooker juices. In a small bowl, mix cornstarch with 2 tablespoons cold water. Stir into the liquid, turn the slow cooker to High, and cook 10–15 minutes until slightly thickened.

Serve. Return sliced brisket to the slow cooker to soak in the sauce, or spoon the sauce over the meat on a platter. Garnish with parsley and serve with mashed potatoes, polenta, roasted vegetables, or rolls.