Add in the mushrooms and rest of the ingredients to the sauted veggies and mix until well combined. Allow the mixture to cool. In the meantime, defrost the puff pastry and lay it out flat on a greased baking tray.
If you want to make one large wellington, you can roll them together or place them side by side to make one large sheet! I made two wellingtons with smaller pastry sheets.
Add in half or all the filling, depending on your pastry size, along the centre & form a log shape with your hands.
Make the same amount of slits on each side of the pastry, about an inch distance apart and quarter of the way on each side
Fold up the ends of each side of the log, then fold each of the slits of the pastry over the middle of the filling in a diagonal crisscross pattern.
Brush with vegan egg wash (one part each oil and plat-based milk) and bake 180C for 45 minutes to an hour until golden.