Place the corned beef brisket into the slow cooker and sprinkle with the seasoning packet or add peppercorns, mustard seeds, and allspice berries.
Add the quartered onion and minced garlic to the slow cooker around the meat.
Pour in the beef broth and add the apple cider vinegar, ensuring the brisket is submerged in liquid. Add more broth or water if necessary.
Cover and cook on low for 8 hours or until the corned beef is tender and easily shreds with a fork.
After about 7 hours of cooking, add the carrots, potatoes, and cabbage wedges on top of the meat. Sprinkle with salt if desired.
Cover again and continue cooking for the last hour until the vegetables are tender.
Once finished, carefully remove the corned beef and let it rest for a few minutes before slicing against the grain into thick pieces.
Serve the sliced corned beef with vegetables on the side and ladle some broth over the top.
For a traditional touch, serve with mustard or horseradish.
Store leftovers in an airtight container in the refrigerator for up to 3 days.