Trim the mushrooms. Cook the onion and sage leaves in the oil over low heat for 10 minutes, in a fairly deep, very wide frying pan or large, shallow saucepan. Stir frequently. Turn up the heat and add the mushrooms. Sprinkle with a pinch of salt and a generous amount of freshly ground pepper and saute for 2 - 3 minutes.

Dilute the rich gravy with 2 tbsp boiling water and add to the mushrooms, together with the tinned tomatoes. Stir, reduce the heat, cover and simmer gently for about 10 minutes or until the mushrooms are tender, stirring occasionally. Keep hot.

A few minutes before it is time to serve this dish, add the potato gnocchi to a very large saucepan of fast boiling salted water and as soon as they bob up to the surface, drain well.

While they are cooking, take the hot sauce off the heat, stir in the garlic, parsley, Parmesan cheese, and the solid butter; as soon as the butter has melted, pour the sauce over the gnocchi and serve at once.