Place corned beef fat side up in slow cooker and sprinkle with spice packet.
Pour beef broth around sides until liquid reaches halfway up the meat.
Cover and cook on low for 5 hours.
Add onion, carrots, and potatoes around the meat.
Cover and cook on low for 1 hour.
Cut cabbage into 6-8 wedges and nestle around other vegetables.
Cover and cook on low for 2 hours until cabbage is tender.
Remove corned beef to cutting board and rest 5 minutes.
Trim excess fat and slice against the grain into 1/4 inch slices.
Remove vegetables with slotted spoon and arrange on platter with sliced beef.