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.