How to Make It
In a large saucepan, cover the potatoes with cool water and season generously with salt. Set the pan over high heat and bring to a boil. Boil the potatoes over moderately high heat until tender, about 20 minutes. Drain well then transfer the potatoes back to the pan.
Meanwhile, in a small glass bowl, heat the milk with the butter until the butter is melted, about 1 minute. Pour the milk mixture into the potatoes, and using a potato masher, mash the potatoes until smooth. Season with salt and pepper, fold in the spinach and serve.