Pat the brisket dry with paper towels. In a small bowl, mix salt, pepper, paprika, garlic powder, onion powder, and thyme. Rub the seasoning evenly over all sides of the brisket.
Heat olive oil in a large skillet over medium-high heat. Sear the brisket for 2–3 minutes per side until nicely browned. Transfer to a slow cooker.
Scatter the sliced onions and minced garlic around and on top of the brisket.
In a bowl, whisk together beef broth, tomato paste, Worcestershire sauce, brown sugar, and apple cider vinegar. Pour the mixture over the brisket. Add bay leaves.
Cover and cook: LOW: 8–10 hours (best for ultra-tender brisket) HIGH: 4–5 hours. The brisket is done when it’s fork-tender and easily pulls apart.
Remove brisket from the slow cooker and let it rest for 10 minutes. Slice against the grain or shred, depending on preference.
Remove bay leaves. Whisk cornstarch and water together, stir into the hot cooking liquid, and cook on HIGH for 10–15 minutes until thickened.
Spoon the sauce over the brisket and serve hot.
