Melt the butter in a saucepan, add the flour and mix. Add the cold milk little by little while stirring. Add salt, pepper and nutmeg. When bechamel becomes a little thick add mushrooms and remove from heat.
Sauté the chicken pieces in the oil and add to the bechamel sauce.
Place a layer of Warda lasagna leaf in the bottom of a buttered baking dish, top with a layer of chicken and mushroom bechamel sauce, add a layer of Gruyere and repeat the process 3 times. Put in the oven 40 minutes.