Heat 2 tablespoons of the olive oil in a frypan and cook the onions 10 minutes or until soft. Set aside.
Heat the remaining oil and brown the beef well on all sides, about 10 minutes. Season with salt.
Pour the wine into a saucepan and reduce it by half over medium. Add half the fried onions and stir well. Add the tomato sauce and beef and stir well. Bring to a boil, lower heat to a simmer and cover. Cook at a low simmer 2 hours, making sure it doesn't get too thick. Add a little water if necessary.
Reheat the remaining onions in the frypan and add the mushrooms. Saute a few minutes until mushrooms are just softening and season.
30 minutes before the end of the sauce cooking time, add the mushrooms, stir and let simmer.
Cook the tagliatelle in boiling salted water until al dente. Serve with the meat sauce and a sprinkling of grated Parmesan.