Preheat your oven to 300°F (150°C).

In a large, heavy-bottomed pot or Dutch oven, heat the olive oil over medium heat.

Season the brisket with salt and pepper on both sides. Once the oil is hot, sear the brisket for about 4-5 minutes on each side until browned. Remove the brisket from the pot and set aside.

In the same pot, add the chopped onion and garlic. Sauté until the onion becomes translucent, about 3-4 minutes.

Add the balsamic vinegar and beef broth to the pot, scraping up any browned bits from the bottom.

Stir in the chopped carrots, parsnips, celery, and fresh tarragon.

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 3-4 hours, or until the brisket is tender and easily pulls apart with a fork.

Once done, remove the brisket from the pot and let it rest for about 15 minutes before slicing.

If you prefer a thicker gravy, mix the cornstarch and water in a small bowl until smooth. Bring the cooking liquid in the pot to a simmer over medium heat and whisk in the cornstarch mixture. Cook until thickened, about 2-3 minutes.

Slice the brisket against the grain and serve with the root vegetable gravy drizzled on top.