In a small saucepan, melt the margarine over medium heat. Add the flour, stirring well. Salt to taste. Add the milk and stir well until it thickens.
Add the nutmeg.

Cut the mushrooms and the onion in thin slices.
In a saucepan, saute the onion until it is caramelized. Add the mushrooms.
Mince the parsley and basil, and add them to the saucepan. Add the bechamel.

In a baking pan, lay 1 pastry sheet and smear it with a little oil. Repeat with 2 more sheets.
Spread the stuffing. Cover with another 3 sheets in the same way.
Slightly pierce the surface and smear a little more oil.
Bake at the oven at 180°C with extra heat on the oven elements, until golden brown.
Leave it to cool down, to thicken, and serve.