Start by patting your corned beef dry with paper towels. Heat a large skillet over medium-high heat and add oil. Once shimmering, sear the corned beef for 2-4 minutes on each side until golden-brown.

Lower the heat and pour in the chosen broth, followed by mustard. Scrape the skillet's bottom to lift browned bits, then pour this over the corned beef in the slow cooker.

Add smashed garlic, halved onion, peppercorns, and thyme. Tuck in bay leaves and sprinkle with celery seed.

Cover the slow cooker and set to low heat. Cook gently for 6-7 hours until fork-tender.

After 6-7 hours, add carrot chunks and continue to cook for an additional 1-2 hours until tender.

Sauté cabbage wedges in butter over medium heat for 5-7 minutes until caramelized. Transfer to slow cooker for the last 30-60 minutes.

Remove corned beef from the slow cooker and let it rest for 10 minutes before slicing against the grain.

Serve the sliced corned beef alongside carrots and cabbage with a ladle of braising liquid. Garnish with parsley and chives, and serve with horseradish sauce and roasted potatoes.