Boil 1 kg of peeled and cubed potatoes in a large pot of salted water for 15-20 minutes, or until they are tender and easily pierced with a fork.
While the potatoes are boiling, heat 2 tablespoons of butter in a pan over medium heat. Add 1 finely chopped onion and cook for 5 minutes, or until softened.
Add 500 grams of chopped greens (such as kale or cabbage) to the pan and cook for another 5-7 minutes, or until the greens are wilted and tender.
Drain the boiled potatoes and return them to the pot. Add 100 grams of butter, 150 ml of warm milk, and salt and pepper to taste. Mash the potatoes until they are smooth and creamy.
Stir the cooked greens and onions into the mashed potatoes, mixing well to combine. Adjust the seasoning if necessary.
Transfer the colcannon to a serving dish and make a small well in the center. Place a knob of butter in the well and allow it to melt before serving.