Preheat your oven to 300°F (150°C).

In a large, oven-safe pot, heat the olive oil over medium-high heat. Season the brisket with salt and pepper on both sides. Once the oil is hot, add the brisket and sear for about 4-5 minutes on each side, until browned. Remove the brisket from the pot and set it aside.

In the same pot, add the diced onion and cook for about 5 minutes, or until softened. Add the minced garlic and continue to sauté for another 1-2 minutes, until fragrant.

In a bowl, combine the pomegranate juice, balsamic vinegar, brown sugar, and dried tarragon. Stir until the sugar is dissolved.

Return the brisket to the pot and pour the pomegranate-balsamic glaze over it. Add the beef broth, ensuring the brisket is mostly submerged in the liquid.

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 shredded with a fork. Check occasionally and add more broth if necessary to keep the liquid level up.

Once the brisket is done, remove it from the pot and let it rest for 15 minutes. Meanwhile, strain the cooking liquid into a saucepan, discarding the solids. Bring the liquid to a gentle simmer over medium heat. If you prefer a thicker gravy, mix the cornstarch with a little cold water to create a slurry, then whisk it into the simmering liquid until it thickens to your desired consistency.

Slice the brisket against the grain and arrange it on a serving platter. Drizzle the velvety pan gravy over the brisket and garnish with fresh tarragon leaves.