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.