Step 1: Pat the brisket dry with paper towels. In a small bowl, combine brown sugar, smoked paprika, garlic powder, onion powder, black pepper, kosher salt, and cayenne pepper (if using) to create your dry rub. Rub the mixture generously over all sides of the brisket.
Step 2: (Optional, but recommended for flavor) Heat olive oil in a large, heavy-bottomed skillet or Dutch oven over medium-high heat. Sear the brisket on all sides until a deep, golden-brown crust forms, about 3-5 minutes per side. Transfer the seared brisket to your slow cooker.
Step 3: Add the chopped yellow onion and minced garlic around and on top of the brisket in the slow cooker. Pour in the beef broth, Worcestershire sauce, apple cider vinegar, soy sauce (if using), and liquid smoke (if using).
Step 4: Cover the slow cooker and cook on the LOW setting for 8-10 hours, or on the HIGH setting for 4-6 hours. The brisket is done when it is fork-tender and easily shreds apart.
Step 5: Carefully remove the brisket from the slow cooker and place it on a cutting board. Tent loosely with foil and let it rest for 15-20 minutes. This allows the juices to redistribute, ensuring maximum tenderness and moisture.
Step 6: While the brisket rests, skim any excess fat from the cooking liquid in the slow cooker. You can thicken the remaining liquid into a gravy by transferring it to a saucepan, bringing to a simmer, and whisking in a cornstarch slurry (1 tbsp cornstarch mixed with 2 tbsp cold water) until desired thickness. Alternatively, continue to simmer in the slow cooker on high if your model allows for thickening.
Step 7: Slice the brisket against the grain for serving, or shred it with two forks for pulled brisket. Serve with the rich gravy and your favorite side dishes. Garnish with fresh parsley or cilantro if desired.