Preheat your oven to 325°F (163°C).

In a large Dutch oven or heavy pot, heat the olive oil over medium-high heat. Season the brisket generously with salt and pepper.

Once the oil is hot, add the brisket to the pot, searing it on all sides until browned, about 4-5 minutes per side. Remove the brisket and set it aside.

In the same pot, add the sliced onions and cook until they are translucent, about 5 minutes. Add the minced garlic and fresh thyme, stirring until fragrant, about 1 minute.

Pour in the balsamic vinegar, scraping the bottom of the pot to deglaze and incorporate any browned bits. Stir in the beef broth, brown sugar, and Worcestershire sauce.

Return the brisket to the pot, along with the diced carrots, parsnips, and potatoes. Bring the mixture to a simmer.

Cover the pot with a lid and transfer it to the preheated oven. Braise the brisket for 3 to 4 hours, or until it is fork-tender.

Remove the pot from the oven and let the brisket rest for 15-20 minutes before slicing.

If desired, strain the gravy from the pot into a saucepan, and simmer over medium heat to thicken. Adjust seasoning with salt and pepper.

Serve the sliced brisket drizzled with the rich root vegetable gravy, garnished with fresh thyme sprigs.