Bring a large saucepan of salted water to a boil. In a large, deep skillet, heat the olive oil. Add the pork and onion and cook over high heat, stirring occasionally, until the onion is lightly browned, about 5 minutes. Add the mushrooms, season lightly with salt and cook over moderate heat, stirring occasionally, until the mushroom liquid has evaporated and the mushrooms are browned, about 8 minutes. Stir in the flour and cook for 1 minute. Add the stock and cook, scraping up the browned bits from the bottom of the pan, until the sauce is slightly thickened, about 1 minute. Remove from the heat.
Cook the pappardelle in the boiling water until al dente. Drain, reserving 1/4 cup of the cooking water. Add the pappardelle and the reserved water to the skillet along with the sour cream and stir well. Simmer for 1 minute. Garnish with parsley and serve right away.