I've been making corned beef in the crockpot for.. about 16 years and I do it a bit differently. I dump the corned beef into the crock pot (I do not use point cut, too much fat), and put in about 2 cups of water tops. It never goes over the corned beef. put the seasoning packet in and grind some fresh pepper. let it cook on high for about 8 hours, and then move it to low or warm. Cut chunks of cabbage up and put in, by this time, the corned beef is covered in its own juices and the cabbage will soak up some of this. Let it simmer for about an hour or so, just until the cabbage is cooked, but not slimy. You want the cabbage to have a little crunch still
Boom, done. No onion, no potato, just cabbage and corned beef. With Flat cut, you'll have a lot less fat to deal with also.
I made corned beef on Sunday and just finished up the last bit today with some scrambled eggs. It goes well with eggs.