Boil the potatoes for about 15-20 min (until soft) and the cabbage for about 10 min. You want the cabbage a little softer than if you were eating it plain.
Mash everything together. Add butter, cream, and milk as desired to get richness and adjust the thickness. Add salt and pepper to taste.
This is a fantastic side dish for corned beef, so if you're boiling a corned beef, put everything else in the same pot when the corned beef is nearly finished. You want about 15 min for the potatoes and about 5 min for the cabbage. You'll get a delicious flavor from the corned beef fat and spices…and of course the result won't be vegetarian.
This is also a great way to use up leftovers.