I have just made this for the second time from Hugh Fearnley Whittingstalls River Cottage Veg Everyday. It is brilliant and uses Orzo pasta and I will use this again and again. (Page 258)
Google it as it is too long to put on here but use chestnut or similar mushrooms and use 1 tablespoon of balsamic vinegar not a teaspoon. Don't cook the pasta till the mushrooms are almost done as they take a lot longer than the pasta to do to the full.
It is a very tasty dish and well worth making. (I bought our pasta from Tesco).