Wash and dry the chicken. Salt it just a little because the cheeses themselves are salty. Stir up a mixture from all of the cheeses and fill the chicken.
It's best to sew it closed after that. Put it in an oiled tray and add the cleaned mushrooms.
Pour on the melted butter and cover with foil. Bake at 392°F (200 °C) for 1 hour, then lower the heat to 356°F (180 °C).
Remove the foil and bake another hour. Pour in about 2/3 cup (150 ml) water and the Bechamel sauce. Return to the oven for another 5 min.