(This is an Irish dish. It takes a little time, but I think it is well worth the extra effort.)
several large potatoes (one per person is good)
2 medium leeks, washed well and sliced
about 1/4 a head of cabbage or equal amount of kale, washed and chopped
1 cup milk
Boil the potatoes in a pot of boiling water until they are tender, as for mashed potatoes. Bring another pan of water to the boil and boil the cabbage. In a smaller saucepan, heat the milk (do not boil it) and simmer the leeks in it for about ten minutes.
Meanwhile, mash the potatoes well (you can peel them or not, as you like). When they are mashed, add the drained cabbage and stir it in. Then, add the hot milk and leeks, and beat the mixture with a wooden spoon just like for mashed potatoes. You can add a little more milk if necessary, and butter for flavor. It should be of about the same consistency as mashed potatoes.
The traditional way to eat this is to make a well in the middle of your colcannon, and add melted butter to the well. You dip forkfuls of it into the well of butter and enjoy. It's quite tasty like that, but it would be healthier (and almost as tasty!) to skip the melted butter.