Slice onions and place on the bottom of the slow cooker. Add corned beef brisket on top. Add enough water almost to cover the brisket. Cook on low for 10 hours.
3-4 hours into cooking, add the potatoes and carrots. Flip over brisket at this time as well.
Eight hours into cooking (2 hours before finishing), add the cabbage cut into 8-12 pieces. You may need to scoop out some water to get it to fit.
Make sauce for the meal by combining the horseradish and mayo and serve. It's done when it easily shreds apart with a fork and reaches an internal temperature of 155F. (It will likely be much hotter than that, though, when it's done.)
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Notes
*nutrition calculation does not include sauce To store corned beef and cabbage: Store corned beef and cabbage in the refrigerator for up to five days after cooking. Reheat in the microwave. To freeze corned beef and cabbage: You can freeze corned beef and cabbage after cooking. However, I don't think the texture will be that great when you defrost it. Your veggies and beef will be very tender when they are done cooking. Freezing and then defrosting will make everything pretty mushy. The beef would be okay to freeze on its own, but not the veggies.