Boil the potatoesYou peel the potatoes if you want a smooth result, or scrub them and leave the skins on for a more textured bowl. You cut them into roughly equal pieces so they cook at the same pace. You place the chunks in your large pot and cover them with cold water by about an inch. You bring the pot to a boil over medium-high heat, then reduce to a steady, rolling boil. You cook for fifteen to twenty minutes until a fork slides into the thickest piece without resistance. You start in cold water on purpose. Dropping potatoes into already-boiling water cooks the outside before the center catches up, and you get soft edges with crunchy middles that taste terrible in mash.
You drain the potatoes in the colander as soon as they turn tender. You let them sit and steam for two full minutes before you mash anything. Surface water that evaporates now cannot thin your mash later.
Cook the greensWhile the potatoes boil, you melt the butter in a separate large pot over medium heat until it foams. You add the chopped kale or shredded green cabbage and stir to coat every piece. You cook the greens for three to four minutes until they wilt and soften but still carry a little color and texture. You add most of the sliced green onions and stir for another minute so they soften slightly without losing their bite. You hold back a small pinch of raw onion for the garnish. You never skip the butter in this step. Steaming the greens in water produces flat, wet leaves that make the whole dish taste watery and underdone.
Get mashingYou tip the drained potatoes directly into the pot with the greens. You pour in the milk and add a generous pinch of salt and several cracks of black pepper. You press the masher down and work through the pot until the potatoes break down and merge with the cabbage into a fluffy, green-streaked mash. You stop before every last lump disappears, because small lumps taste honest and keep the texture interesting. You taste a spoonful before you move the pot off the stove. You ask whether it tastes rich, seasoned, and balanced. If the answer hesitates, you adjust.
Serve it upYou transfer the colcannon to a warm serving bowl. You scatter the reserved sliced green onions over the top. You press a well into the center of the mash with a spoon and drop in a pat of cold butter. If the butter does not begin melting into a golden pool within thirty seconds, the mash cooled too fast and needs a quick stir over low heat before it reaches the table.
