Pat the 4-5 pound beef brisket dry with paper towels to remove excess moisture. Generously season both sides with kosher salt, black pepper, smoked paprika, onion powder, and garlic powder.

In a large skillet, heat olive oil over medium-high heat until shimmering. Add the seasoned brisket and sear for 3-4 minutes on each side until browned.

Transfer the brisket to a plate. In the same skillet, add the sliced onion and minced garlic. Sauté for 3-4 minutes. Pour in beef broth, balsamic vinegar, brown sugar, tomato paste, and Worcestershire sauce. Simmer for 2 minutes.

Place the seared brisket into the slow cooker and pour the prepared sauce over it. Add bay leaves and thyme. Cook on low for 8 to 10 hours.

Remove the brisket from the slow cooker and let it rest for 15-20 minutes. Slice against the grain and serve with sauce.