Peel and quarter the potatoes. Add to a large pot and cover with water. Bring to a boil over medium-high heat. Lower heat to medium low and cook the potatoes until fork tender, about 20 to 25 minutes.

Drain the potatoes and return to the pot, then cover and set aside.

Heat a large cast iron pan or skillet over medium-low heat. Add the olive oil and heat through. Add the whites of the green onions followed by the cabbage to the pan. Sauté the cabbage has wilted slightly, stirring frequently, about 5 minutes.

Add the kale and continue to sauté until both the cabbage and kale are tender and have reduced in size, about an additional 5 minutes.

Add the milk and 3 tablespoons of the butter to the potatoes and mash with a potato masher or fork. Add the greens and continue to mash until combined. Season to taste with salt.

Transfer the colcannon to a serving bowl. Make a well on the top and add the remaining butter and garnish with the green onions.