Rub 2 tbs of Dijon mustard on the top (fatty side) of the corned beef. Mix the spice packet with minced garlic, brown sugar, black pepper, and fennel seed and sprinkle on the top of the mustard.
Pour the apple juice (or apple cider) and apple cider vinegar into the slow cooker. Add prepared corned beef, seasoned side up, to the pot of your slow cooker. Cook on low for 6-7 hours.
Just before serving, place corned beef on a small foil-lined sheet pan and place under a broiler (6-7 inches away) for 5-6 minutes to caramelize and crisp the top. Keep an eye on it, as it browns quickly.
