Colcannon is hearty and richly satisfying, a delicious and homey blend of creamy potatoes and kale. Some recipes omit the green onions, but I find they lend a little brightness to the dish. Use a medium-starch potato, such as Yukon Gold, if you can find them. Russets also work well too.
Arrange the potatoes in a medium pot, and then cover with cold water by an inch. Bring it all to a boil over medium-high heat. Boil the potatoes until tender, about 15 minutes.
While the potatoes cook, tip the green onions into a small saucepan, and then cover them with milk. Warm the milk and green onions together over medium-low heat.
When the potatoes are tender, drain them in a colander and return them to the cooking pot. Pour the milk and green onions over the potatoes, and then dump in the steamed kale. Mash them all together until smooth and creamy, adjusting the seasoning as needed with salt.
To serve, transfer the colcannon to a serving bowl, and make a well in the center of the mash with the back of a spoon. Drop the butter into the well, and let it melt in the heat of the potatoes. Serve hot.
Store any leftovers in the fridge for up to 5 days.