Cook the tagliatelle in salted water until al dente. Peel and finely dice the onion and garlic. Chop the parsley. Cut the mushrooms into large slices.
Heat the oil in a pan, sauté the onion and garlic, add the mushrooms and sauté briefly. Stir in the cream, soup, mascarpone and balsamic vinegar. Simmer for approx. 10 minutes until creamy. Stir occasionally.
Stir the cornflour into 3 tbsp. cold water. Stir to thicken the sauce and flavour with salt and pepper. Stir in the parsley. Drain the tagliatelle and mix into the mushroom sauce while still dripping wet.