In a bowl, combine balsamic vinegar, olive oil, minced garlic, chopped rosemary, brown sugar, salt, and black pepper. Whisk until well combined.

Place the beef brisket in a large resealable plastic bag or a shallow dish. Pour the marinade over the brisket, ensuring it is well coated. Seal the bag or cover the dish and refrigerate for at least 4 hours, preferably overnight.

When ready to cook, preheat your oven to 325°F (160°C).

In a large Dutch oven or oven-safe pot, heat a tablespoon of olive oil over medium-high heat. Remove the brisket from the marinade and sear it in the pot for about 4-5 minutes on each side until browned. Remove the brisket and set aside.

In the same pot, add diced onion, carrots, and parsnips. Sauté for about 5-7 minutes until the onions are translucent.

Pour in the beef broth and Worcestershire sauce, scraping up any browned bits from the bottom of the pot.

Return the brisket to the pot, nestling it among the vegetables. Cover the pot with a lid and transfer it to the preheated oven. Braise for 3 to 4 hours, or until the brisket is tender and easily shredded with a fork.

If you prefer a thicker gravy, remove the brisket and vegetables from the pot and keep warm. Mix cornstarch with a little cold water to create a slurry and stir it into the pot. Bring the liquid to a boil over medium heat until thickened.

Slice the brisket against the grain and serve with the root vegetables and rich gravy. Garnish with fresh rosemary sprigs if desired.