Prepare the Rub: In a small bowl, whisk together the smoked paprika, brown sugar, garlic powder, onion powder, salt, black pepper, and cayenne.
Season the Beef: Pat the brisket completely dry with paper towels. Generously rub the spice mixture all over the brisket, pressing it firmly into the meat.
Sear (Optional but Recommended): Heat olive oil in a large skillet over medium-high heat. Sear the brisket for 2-3 minutes on each side until a brown crust forms. Remove from heat.
Layer the Slow Cooker: Place the sliced onions and minced garlic at the bottom of the slow cooker insert. Lay the seasoned brisket on top of the onions, fat side facing up.
Add Liquids: In a separate bowl or measuring cup, mix the beef broth, Worcestershire sauce, soy sauce, and apple cider vinegar. Pour the liquid around the edges of the brisket (do not pour directly on top to avoid washing off the spices).
Cook: Cover the slow cooker and cook on LOW for 8 to 10 hours, or until the meat is incredibly tender and easily yields to a fork.
Rest and Slice: Carefully remove the brisket to a cutting board. Tent loosely with foil and let it rest for 15-20 minutes. Be sure to slice against the grain for maximum tenderness.
Serve: Skim the fat from the juices left in the slow cooker and serve the liquid as a warm gravy over the sliced beef. Enjoy!