How to Make It
In a large pot of boiling salted water, cook the pappardelle until al dente.
Meanwhile, in a medium heatproof bowl, cover the porcini with the boiling water and let stand until softened, about 5 minutes. Lift the mushrooms from the water and coarsely chop them. Reserve the soaking liquid.
In a large saucepan, heat the olive oil. Add the porcini, pancetta, celery, carrot, onion and thyme sprigs and cook over moderately high heat, stirring often, until the vegetables start to brown, about 5 minutes. Add the ground beef and turkey, season with salt and pepper and cook, breaking up the meat with a wooden spoon, until no pink remains, about 5 minutes. Add the red wine and boil over high heat until reduced by half, about 3 minutes. Add the tomato puree. Pour in the reserved mushroom soaking liquid, stopping before you reach the grit at the bottom. Bring to a boil and simmer over moderate heat, stirring occasionally, until thickened and flavorful, about 12 minutes. Discard the thyme sprigs.
Drain the pappardelle and return it to the pot. Toss with the butter and then add half of the ragù and toss well. Season with salt and pepper and toss with the 1/4 cup of Parmesan. Transfer the pasta to bowls, top with the remaining ragù and serve. Pass additional Parmesan at the table.