Boil the potatoes: Put the potatoes in a medium pot and cover with cold water by ½ inch. Add 2 tablespoons of salt, and bring to a boil until the potatoes are fork-tender, 15 to 20 minutes. Drain in a colander
Saute the spinach and the green onions with butter until well done and onions are somewhat translucent
Mash the potatoes with sour cream and greens mixture
Add salt to taste and serve hot, with a knob of butter in the center