For the sauce:
Soak the dry mushrooms in warm water for 30 minutes.
In the meantime grind the walnuts using a pestle and mortar or an electric mixer.
Very finely chop the garlic clove and parsley, add to the walnuts, season well with salt and pepper and stir thoroughly.
Heat up in a small pan the oil along with the butter. Once it starts to sizzle add the chopped onion and fry until lightly golden in colour, making sure not to burn it.
Drain the mushrooms and gently squeeze out the excess of water, then chop.
Add the mushrooms into the pan, give everything a stir and let it cook on a low heat for about 20 minutes. You may have to occasionally add a little bit of water once the onion and mushroom mix starts to stick to the bottom.
Next add the walnut, garlic and parsley mixture. Give everything another stir and fry for about 5 minutes or so, allowing the garlic to release its aroma.
Pour in the cream, stir again and keep the sauce on a low heat. Taste and adjust the seasoning.
For the pasta:
Bring a large pot of salted water to the boil.
Add the pasta and boil until al dente.
Drain the pasta reserving on the side some water from cooking the pasta.
Add the pasta into the pan with the sauce and toss everything together adding some reserved water if the sauce turns too dry.
Add a little bit of lemon juice and freshly chopped parsley. Taste and season again if needed.
Serve immediately garnishing the dish with grated parmesan.