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.)
Video
Notes
*nutrition calculation does not include sauce **see post for swaps and substitution ideas