Rinse and pat dry the corned beef brisket to remove excess salt. Set aside.
Place the onion slices and minced garlic in an even layer at the bottom of a large 6- to 8-quart slow cooker.
Lay the corned beef on top of the onions, fat side up, and sprinkle with the seasoning packet and bay leaves.
Add the red potatoes and carrots around the beef. Pour in your chosen liquid until the vegetables are partially submerged.
Cover and cook on low for 8-10 hours, or until the brisket is tender.
About 2 hours before the cooking time ends, add the cabbage wedges on top of the beef and vegetables. Replace the lid and continue cooking.
Once done, transfer the brisket to a cutting board and let rest for 5-10 minutes. Meanwhile, use a slotted spoon to remove the vegetables and transfer them to a serving platter.
Slice the beef against the grain and arrange it alongside the vegetables. Drizzle with some of the cooking liquid for added flavor.
Garnish with fresh parsley and serve with mustard or horseradish sauce, if desired.