The base of Duxelles is invariably chopped mushrooms but it can be prepared with any edible fungus. Heat 30 g (1 oz) butter in a pan with an equal amount of oil, add 20 g (⅔ oz) each of chopped onion and chopped shallot and cook gently for a few minutes. Add 250 g (9 oz) finely chopped trimmings and stalks of mushrooms, which have been firmly squeezed to remove all the vegetable juices. Cook gently until the complete evaporation of all the moisture. Season with salt and pepper and finally add a pinch of chopped parsley. Place the mixture in a basin and cover with a buttered paper. This dry Duxelles is used in a number of preparations.