First we make the bechamel that we need for the filling. To do this, cook the butter with the flour for five minutes and add the cream and milk in three parts. Cooking everything together so that the flour stops tasting and we reserve warm.
Sauté the chopped onion in a frying pan with a splash of olive oil for ten minutes and add the chopped mushroomscooking five more minutes over medium-high heat.
Add the shredded chicken, the soy sauce and the bechamel and we cook the whole set for seven minutes more (this is the trick to make them very syrupy).
We correct salt and pepper and cool in a tray covered with transparent film in contact with the filling. if we want, we have in a pastry bag to fill easier the cannelloni.
Then when the cannelloni plates are hydratedarrange them on a board, place the filling along one end with the help of the pastry bag and wrap the cannelloni until it is well closed.
In an oven tray, arrange four or five tablespoons of bechamel at the base of the tray and place the cannelloni neatly on top. We cover (totally cover) the cannelloni with the bechamel we had made and spread the gratin cheese on top.
We put in the oven about 15 minutes with grill and fan at 225 ºC until they are well au gratin and serve very hot in a source or in individual clay bowls.