Put the potatoes in a medium pot and cover with cold water by at least an inch. Add 2 tablespoons of salt, and bring to a boil. Boil until the potatoes are fork tender, 15 to 20 minutes. Drain in a colander.
Return the pot to the stove and set over medium-high heat. Melt the butter in the pot and once it's hot, add the greens. Cook the greens for 3-4 minutes, or until they are wilted and have given off some of their water. Add the green onions and cook 1 minute more.
Pour in the milk or cream, mix well, and add the potatoes. Reduce the heat to medium. Use a fork or potato masher and mash the potatoes, mixing them up with the greens. Add salt to taste and serve hot, with a knob of butter in the center.