In a small bowl, mix together the brown sugar, smoked paprika, garlic powder, onion powder, chili powder, salt, and black pepper. Rub this spice mixture generously all over the brisket on both sides.
Spread the sliced onion rings and smashed garlic cloves in an even layer on the bottom of the slow cooker. This creates a flavorful bed for the brisket to sit on.
In a small bowl, whisk together the beef broth, worcestershire sauce, and tomato paste. Pour this liquid into the slow cooker over the onions.
Place the seasoned brisket fat-side up on top of the onions. If necessary, cut the brisket in half to fit your slow cooker.
Cover and cook on low for 9 to 10 hours. Do not open the lid during cooking. The brisket is done when it is fork-tender and pulls apart with almost no resistance.
Carefully remove the brisket from the slow cooker and let it rest on a cutting board for 10 minutes. Slice against the grain into thin slices.
Strain the cooking liquid and skim off excess fat. Spoon the onion gravy over the sliced brisket and serve. This pairs wonderfully with roasted vegetables or creamy coleslaw.