Cool white sauce slightly and flavor well with salt, pepper, nutmeg, then beat in the eggs, one by one.
Line a jelly roll pan with lightweight aluminum foil.
Butter it and pour in the egg mixture.
Bake for 20 minutes or until set but not brown at 350F (180C).
Invert on a length of buttered foil and peel off the bottom foil.
Spread with Mushroom Filling and roll up jelly roll style.