Join the two sheets of puff pastry together. Unwrap the beef roll from the cling wrap and place the beef on the duxelle, rolling it up tightly & Place the Wellington seam-side down on a lined tray.
If not doing a lattice top, brush the top of the Wellington with egg wash and it is ready to bake. Sprinkle dough with a bit of kosher salt after brushing with egg wash. You may use leftover scraps of pastry to make a pretty design on top,
If adding a decorative lattice top, wait to add the egg wash to the base pastry until just before you put the lattice top on. Using another pastry piece of cold puff pastry, on a lightly floured surface, carefully roll a lattice roller lengthwise over the pastry using moderate pressure to cut through it thoroughly. Try to gently pull the lattice apart. If it is not cut all the way through, follow the lines gently with a paring knife to separate it. Brush thoroughly with the beaten eggs.
Bake for 35 minutes or until the puff pastry is golden brown and the internal temperature of the beef is 57c for medium-rare.