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.