Place the potato chunks into a large pot, ensuring they are covered with cold water, leaving about an inch of water above the potatoes. Bring the water to a rolling boil and cook the potatoes for 15–20 minutes, or until they are tender enough to pierce easily with a fork.

While the potatoes are cooking, melt the butter in a separate large pot over medium heat. Add the chopped kale or cabbage to the pot and sauté for 3–4 minutes until it wilts and softens. Stir in the green onions, reserving a small portion for garnish, and cook for an additional minute to release their flavor.

Once the potatoes are done, drain them thoroughly in a colander. Transfer the hot potatoes to the pot with the kale mixture. Pour in the milk, season generously with salt and pepper, and begin mashing everything together. Continue mashing until the mixture becomes light, fluffy, and evenly combined.

Scoop the colcannon into a serving dish. For a finishing touch, sprinkle the reserved green onions over the top and add a small pat of butter to melt into the warm mash. Serve immediately and enjoy!