Also known as stamp, colcannon turns potatoes and greens into a full and hearty meal. I like to use cabbage, kale, collards—whatever leftover greens I have. A layer of caramelized fried onion tops the buttery-soft potato mountain.
Melt 3 tablespoons of the butter in a large skillet over medium-high heat. Add the onions and cook until they are golden and caramelized, 15 to 20 minutes.
Meanwhile (if starting with raw potatoes), boil the potatoes in salted boiling water until tender, 10 to 20 minutes. Drain, reserving 1 cup of the cooking water. Peel and discard the skins and mash the potatoes with the milk and 2 tablespoons of the butter. If the potatoes are too thick, add some of the cooking water to achieve a smooth consistency. Season the potatoes with salt and pepper. Stir in the cooked greens. Melt the remaining 3 tablespoons butter. To serve, place a mound of potatoes and greens in a shallow bowl and make a well in the center. Fill with some melted butter and top with some of the fried onions.