These potatoes can be served as a taco filling or as a side dish with fish or meat.
Bring a pot of lightly salted water to a boil. Add potatoes and cook until tender, about 30 minutes. Drain and let cool. Peel potatoes and cut into halves or quarters so pieces are of uniform size.
Melt butter in a large saute pan and cook onion over medium-low heat until transparent, about 7 minutes. Add potatoes and salt to taste, and continue cooking over low heat for 30 minutes. Stir in parsley leaves. Serve hot or warm.