Put the potatoes in a medium pot and cover with cold water by at least an inch. Add salt, and bring to a boil. Boil until the potatoes are fork-tender, about 15 to 20 minutes. Drain in a colander and put in a large bowl or the bowl of a stand mixer.

Add the butter and the milk to the potatoes and either mix with the paddle attachment or mash by hand with a masher.

Boil about 4 cups of water on the stove and get a bowl ready with some ice and cold water and put it next to the stove. Add the chopped kale and boil for 1 minute. Add the chopped onions and boil for 1 more minute. Remove with a slotted spoon and immediately submerge in the ice water.

Drain the kale and onions and press down on them to get rid of most of the water. Mix into the mashed potatoes and stir until well combined.

Serve with a pat of butter on top while still hot.