Put the potatoes into a large pot, with the larger ones on the bottom, and add water to come halfway up the potatoes. Cover the pot and bring to a boil over high heat. When the water begins to boil, carefully drain off about half of it, then return the pot to the heat, cover it again, reduce the heat to low, and let the potatoes steam for about 40 minutes. Turn off the heat, cover the potatoes with a clean, damp tea towel, and let sit for 5 minutes more.

Melt 4 Tbsp of the butter in a large skillet over medium-high heat. Add the kale or assorted greens and cook until just wilted, about 5 minutes.

Combine the milk, scallions, and remaining butter in a medium pot and bring to a simmer over medium heat. Cook for about 2 minutes, then add the greens and stir in well. Remove the pot from the heat, cover, and set aside.

Drain and carefully peel the potatoes, then return them to the pot. Add the greens and their liquid and mash until smooth, leaving a few small lumps in the potatoes. Season to taste with salt and pepper.

To serve in the traditional Irish manner, push the back of a large soup spoon down in the middle of each portion to make a crater, then put a large pat of room-temperature butter into each one to make a “lake.” Diners dip each forkful of colcannon into the butter until its walls are breached.