Boil the potatoes: 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). While potatoes are boiling, sauté the kale and onions with butter. Drain potatoes in a colander. Return the pot to the stove and set over medium-high heat. Mash the potatoes with milk, add kale and onions. Add salt to taste and serve hot, with a dollop of butter in the center.