Note I have said nothing about cabbage yet. It's not time! When the roast is done and resting, then you do the cabbage.
I use a saute pan for the cabbage. I put a couple of good spoonsful of the liquid from the roast into the pan, with just a tiny splash of soy, and heat that up. When it's really hot, add your sliced cabbage and a bit of freshly cracked black pepper and saute it over fairly high heat until it is tender crisp. And that's it for the cabbage.
Time to slice the corned beef roast and eat!