To prepare spinach: Cut tough stalks from spinach. Wash leaves well. Drop leaves into boiling water. Cook for 30 seconds or until leaves are just softened. Drain and refresh in cold water. Pat dry with paper towel and set aside.
If using frozen spinach, thaw. Squeeze out excess water before placing on pastry.
Heat butter in large pan. Add filet of beef. Brown well on all sides. Cool. Retain pan juices.
Preheat oven to 400°F.
Place pastry on a flat surface. Overlap edges by a ½ inch. Press together well to seal. Arrange spinach over pastry leaving a 2 ½ inch border. Spread or crumble cheese over spinach. Place beef in centre of pastry and season – be sparing with the salt as the cheese will impart its own saltiness. Fold in narrow end of pastry. Fold remaining pastry over to enclose beef. Turn seam to underside and place on a lightly greased baking pan. Brush top with egg.
Bake for 25-30 minutes or until puff pastry is golden. Rest, tented with foil, for 10 minutes before slicing.