Place the sliced onion in the bottom of the slow cooker. Rinse corned beef under cool water and reserve spice packet. Place beef in slow cooker on top of onions with the fatty side up. Press the black pepper and contents of the spice packet into the top of the beef, spreading evenly.

Add beef broth, pepper, garlic and bay leaves.

Cover and cook for 10–12 hours on low or 6–8 hours on high. Add carrots and potatoes during the last 4 hours of cooking and add cabbage in the final hour of cooking.

Once cabbage cooks to desired tenderness and brisket is fall-apart tender, remove each vegetable component from the slow cooker into serving bowls. Transfer corned beef to a cutting board and slice against the grain into thin strips.

Arrange sliced corned beef on a serving platter. Strain cooking liquid, discarding solids, including bay leaves. Pour cooking liquid over corned beef and serve.