Begin by patting the beef brisket dry with paper towels to ensure a better sear. In a small bowl, mix together the kosher salt, black pepper, smoked paprika, onion powder, and garlic powder. Generously rub this spice blend over both sides of the brisket for maximum flavor.

Heat the olive oil in a large skillet over medium-high heat. Once the oil is shimmering, carefully add the seasoned brisket and sear it for 3-4 minutes on each side, until golden crust forms.

Using the same skillet, add the sliced onion and minced garlic. Sauté for about 3 minutes until the onion softens and becomes fragrant. Add the beef broth to deglaze the pan and stir in the balsamic vinegar, brown sugar, tomato paste, and Worcestershire sauce, simmering for 2 minutes.

Transfer the seared brisket into the slow cooker. Pour the sauce over the brisket, add the bay leaves and thyme, and cover the slow cooker, cooking on low for 8-10 hours.

As the cooking time nears completion, ensure the brisket is fork-tender. Carefully remove the brisket from the slow cooker and let it rest for 10 minutes before slicing against the grain.