Step 1: Pat the brisket dry thoroughly with paper towels. Trim any excess hard fat, leaving about 1/4 inch of soft fat.
Step 2: In a small bowl, combine the brown sugar, smoked paprika, garlic powder, onion powder, salt, and black pepper. Rub this mixture generously all over the brisket, pressing it firmly to adhere. Allow it to sit at room temperature for 15-30 minutes while you prepare to sear, or refrigerate for a few hours (or overnight) for deeper flavor.
Step 3: Heat 1-2 tablespoons of olive oil in a large skillet or Dutch oven over medium-high heat. Once hot, carefully place the brisket fat-side down in the pan. Sear for 4-5 minutes per side until a deep golden-brown crust forms.
Step 4: Once seared on all sides, carefully transfer the brisket to your slow cooker.
Step 5: Add the sliced onion and minced garlic around the brisket in the slow cooker.
Step 6: Pour the beef broth (and optional Worcestershire sauce) over the brisket and vegetables. You want enough liquid to come about halfway up the sides of the brisket.
Step 7: Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours, or until the brisket is fork-tender. It should be so tender you can easily pull it apart with a fork.
Step 8: Once cooked, carefully remove the brisket from the slow cooker and place it on a cutting board. Tent it loosely with foil and let it rest for at least 15-20 minutes.
Step 9: While the brisket rests, you can skim any excess fat from the cooking liquid in the slow cooker. If desired, to make a gravy, whisk together 1 tablespoon of cornstarch with 2 tablespoons of cold water to form a slurry. Stir this into the hot liquid in the slow cooker and cook on high for 15-20 minutes, stirring occasionally, until thickened.
Step 10: Slice the brisket against the grain for best texture, or shred it using two forks. Serve immediately with the gravy on the side.