Heat butter and half of the oil in a heavy based pot. Add mushrooms and thyme, cooking over a medium high heat until seared. Add a splash of the white wine to deglaze and then set mushrooms aside.
Heat remaining oil in the same pot. Add the leak and garlic, cooking for several minutes until tender. Stir in barley and cook for 1 minute.
Pour in remaining wine and allow to evaporate. Add stock one ladel at a time, ensuring each is absorbed before adding the next. Prior to adding the last of the stock, return mushroom mixture to the pot and stir through.
Stir parmesan through risotto and season to taste. Finish with parsley and creme fraiche.