Season the brisket generously with salt and pepper. In a hot cast‑iron skillet, add 1 tablespoon of olive oil and sear the brisket on all sides until a deep brown crust forms, about 3–4 minutes per side. Transfer the brisket to the slow cooker.

In the same skillet, add the sliced onions, brown sugar, and balsamic vinegar. Cook over medium heat, stirring frequently, until the onions are caramelized and golden, about 10 minutes. Transfer the onions to the slow cooker.

Toss the carrots, parsnips, and potatoes with 1 tablespoon of olive oil, salt, pepper, and thyme. Spread them on a baking sheet and roast in a preheated 425°F oven for 25 minutes, or until they are just tender and lightly browned.

Add the roasted root vegetables to the slow cooker. Pour in the beef broth and red wine, then toss in the bay leaf and whole garlic cloves.

Cover and cook on low for 7 hours (420 minutes), or until the brisket is fork‑tender.

Let the brisket rest for 10 minutes before slicing against the grain. Serve with the caramelized onions and root vegetables. Before serving, use a meat thermometer to ensure the brisket has reached a safe internal temperature.