In a medium saucepan, cover the potatoes with water and bring to a boil. Reduce the heat to moderately high and simmer until tender, about 20 minutes; drain.
Let the potatoes cool slightly, then cut into 1-inch pieces.
In a medium cast-iron skillet, cook the pancetta over moderately high heat for 2 minutes.
Transfer the pancetta to a plate. Melt the butter in the skillet.
Add the potatoes and cook over moderate heat, turning occasionally, until golden and crisp, about 7 minutes.
Add the onion and pancetta, season with salt and pepper and cook until the onion is translucent, about 5 minutes. Stir in the parsley and serve right away.