Preheat your oven to 163 °C (165 °C). Pat the brisket dry with paper towels and generously season all sides with salt and pepper.

In a large Dutch oven or oven-safe pot, arrange the thinly sliced onions evenly on the bottom. Place the seasoned brisket on top of the onions.

In a separate bowl, whisk together the balsamic vinegar, beef broth, packed brown sugar, Worcestershire sauce, minced garlic, and fresh thyme sprigs. Pour this mixture evenly over the brisket.

Cover the Dutch oven tightly with its lid or heavy-duty aluminum foil. If not using a Dutch oven, ensure your baking dish is also tightly covered.

Bake in the preheated oven for 3 to 3.5 hours, or until the brisket is very tender and easily pierced with a fork. The internal temperature should reach at least 91 °C (90 °C).

Remove the brisket from the pot and let it rest on a cutting board for at least 15 minutes. While the brisket rests, strain the cooking liquid into a saucepan, discarding the solids (onions and thyme). Skim off any excess fat from the liquid.

Bring the strained cooking liquid to a simmer over medium heat. Let it reduce slightly for about 5-10 minutes to thicken into a glaze consistency. Taste and adjust seasoning if needed.

Slice the rested brisket against the grain. Serve the sliced brisket drizzled generously with the reduced balsamic glaze. Spoon extra caramelized onions from the pot over the top.