Rinse corned beef under cold water. Pat dry.
Place in slow cooker fat-side up.
Sprinkle spice packet over meat.
Add water or broth until halfway up the meat.
Add bay leaves and garlic to liquid.
Add potatoes, carrots, and onion around meat.
Sprinkle brown sugar on top if using.
Cover. Cook on low 7 to 8 hours or high 4 to 5 hours.
After 6 hours on low or 3 hours on high, add cabbage wedges.
Continue cooking until cabbage is tender and meat is fork-tender.
Remove meat. Rest 10 minutes on cutting board.
Remove vegetables with slotted spoon. Place on platter.
Slice meat against the grain into thin slices.
Arrange meat and vegetables on platter. Serve hot.