Rinse the corned beef brisket under cold water and pat it dry with paper towels. If your corned beef came with a spice packet, set it aside to use later.
Place the sliced onion in the bottom of your slow cooker. Add the minced garlic on top of the onions.
Place the corned beef brisket, fat side up, on top of the onions and garlic.
In a small bowl, whisk together the beef broth (or water), Worcestershire sauce, black pepper, and the contents of the seasoning packet (if using). Pour this liquid mixture over the corned beef.
If using potatoes and carrots, arrange them around the corned beef in the slow cooker.
Cover the slow cooker and cook on LOW for 8-10 hours, or on HIGH for 4-5 hours, until the corned meat is fork-tender. The cooking time will vary depending on your slow cooker and the size of the brisket.
Once cooked, carefully remove the corned beef from the slow cooker and let it rest on a cutting board for about 10 minutes before slicing against the grain.
Serve the sliced corned meat with the cooked vegetables and the flavorful cooking liquid spooned over the top.