Season the tuna and fry very quickly (10 seconds on each side should do it) in a hot frying pan on all sides and both ends to sear it.
Lay out a thick square of cling film about 30cm square and place the sheets of nori seaweed so they overlap. Spread thinly with the mushroom and lentil puree and place the tuna at the bottom of the nori.
Now it's time to roll with it! Take hold of the cling film at the back and slowly wrap the nori and puree over the tuna to make a log. The nori should fully enclose the tuna. Seal the ends of the cling film and place in the fridge for 1-2 hours until set.
Whisk up the two eggs for the egg wash and set aside.
Lightly flour the table top and lay out the puff pastry. Roll out the pastry to 5mm thickness ensuring that the pastry is much larger than the tuna log.
Remove the tuna from the cling film and place in the centre of the pastry, egg wash all sides and slowly fold over to wrap the tuna, trim off any excess pastry and leave to chill for 1 hour.
Adding a lattice is optional, but it does make for a show stopping dish! If you do want to add a lattice, roll out any extra pastry so it is the length of the tuna roll and apply the specialised pastry cutter to create the lattice. Then, egg wash the log and place the lattice over the top.
To cook the wellington place in a preheated oven at 180oC for 25 minutes, or until the pastry has turned golden brown. Leave to rest for 8 minutes before carving into slices and enjoy!