Preheat the oven to 325°F and grab a large Dutch oven or heavy pan.
Season the beef brisket generously with salt and pepper on both sides.
Heat olive oil in the pan and sear the brisket until browned all over. Remove and set aside.
In the same pan, add the onions and cook until golden and soft. Stir in garlic, brown sugar, soy sauce, vinegar, ketchup, and mustard.
Pour in the beef broth and stir to combine, scraping up any browned bits from the bottom.
Return the brisket to the pan, cover it tightly, and transfer to the oven.
Cook for about 3 to 4 hours, basting occasionally, until the beef is fork-tender and the sauce is thick and caramelized.
Shred the brisket using two forks and mix it with the caramelized sauce before serving.