Add the onion, garlic, carrots, and potatoes to the slow cooker. Place the corned beef brisket on top, fat side up. Spread the contents of the seasoning packet evenly over the brisket.
Place the cabbage wedges around the brisket. Pour the water, beef broth, and apple cider vinegar into the slow cooker until the brisket is just covered.
Cover and cook on low for 8-10 hours or on high for 4-5 hours, until the corned beef is tender and easily pierced with a fork.
In a small bowl, whisk together honey, Dijon mustard, whole-grain mustard, and apple cider vinegar.
Preheat your oven’s broiler. Remove the brisket from the slow cooker and place it on a baking sheet lined with foil. Brush the honey mustard glaze generously over the brisket. Broil for 5-7 minutes, until the glaze caramelizes. Watch carefully.
Slice the corned beef against the grain and serve with the cooked vegetables. Drizzle some of the slow cooker broth over the top for extra flavor.
Notes
This dish tastes even better the next day. Store leftovers in the broth to keep the meat moist. Makes great leftovers.Keep the broth from the slow cooker and use it as a stock for future recipes. Place in an airtight container like a mason jar and refrigerate.You can skip the glaze if you would like, but I highly recommend it as it adds more depth of flavor.If your brisket has a lot of fat, I recommend trimming it down before cooking.