Cook the mushrooms in a hot pan with some olive oil, add the thyme and season with salt and freshly ground black pepper. Set aside.
Warm up half the butter in a heavy saucepan and sweat the onion, celery and garlic until soft, add a pinch of salt to help bring out the flavour.
Add the rice, coat the rice in the butter and cook it for a couple of minutes until it begins to sing (pop).
Add the vermouth and allow it to cook until the alcohol has evaporated (the smell of alcohol will have subsided).
Start to gradually add the stock, stirring all the time, ¾ of way through the cooking add the cooked mushrooms and the parsley; the rice will take approximately 20 minutes to cook.
To finish and serve
When the rice is cooked, but still firm turn off the heat then add the butter, Parmesan, stirring all the time and serve. The risotto should be slightly wet and running.