Peel potatoes and cut into 1/2-inch cubes. In a deep 12-inch heavy skillet cook onion in oil over moderate heat, stirring frequently, until golden brown. Add parsley, potatoes, salt, pepper, and water plus additional if necessary to barely over potatoes and simmer, covered, until potatoes are very tender, about 20 minutes.
While potatoes are cooking, in a 6-quart kettle bring 5 quarts salted water to boil for pasta.
Cook pasta in boiling water until al dente and drain in a colander. Return pasta to kettle and add potato mixture, tossing gently to combine well.
