In a heat-resistant container, add the mushrooms and boiling water. Leave to hydrate for 30 minutes.
In a pan over medium heat add the butter and oil, when the butter has melted add the chopped onion and stir until translucent (about 3 minutes). Add the wheat flour and stir for about 3/4 minutes or until it starts to brown and has an almond aroma.
Gradually add the milk, stirring vigorously with a wire whisk to avoid lumps. Add the nutmeg, pepper and salt and cook stirring for 7 minutes until the sauce is very thick.
With the help of a sieve and a clean cloth or a voile, strain the funghi liquid directly into the white sauce.
Wash the pieces of funghi well under running water to remove any impurity. Discard the hard parts, chop the rest and add to the sauce. Cook for another 5 minutes, stirring constantly so it doesn't stick to the bottom of the pan.