Peel and cut the celeriac into 2 cm cubes.
Peel and dice the onion.
Peel and crush the garlic.
Roughly chop the mushrooms.
Destalk the herbs.
Beat the egg together.
Fit the Knife blade to the food processor bowl.
Pre heat the oven to 180°C.
Add Ingredients 1 (celeriac, water) into the saucepan.
Heat over a medium heat.
Boil until soft.
Remove the celeriac with a slotted spoon, leaving the water in the pan, and place in a large bowl.
Add Ingredients 2 (farro) into the pan of hot water.
Simmer for 20 minutes until tender.
Drain and leave to cool.
Roughly mash the celeriac and add to the farro.
Add Ingredients 3 (oil) into the sauté pan.
Add the next 2 items of Ingredients 3(onion, garlic) into the pan.
Cook over a medium heat until soft.
Add remaining items of Ingredients 3 (mushrooms) into the pan.
Increase the heat and allow the mushrooms to reduce in volume and start to colour.
Remove from the heat and add the cooked vegetables to the celeriac.
Add Ingredients 6 (nuts, herbs) into the food processor.
Blend until everything is roughly chopped.
Transfer the contents to the celeriac.
Add the first 2 of Ingredients 7 (flour, butter) into the food processor.
Pulse until the butter is distributed through the flour.
With the machine still running add the rest of Ingredients 7 (egg yolk, water).
Mix until the pastry comes together.
Transfer the pastry to a worksurface and bring together.
Wrap in plastic wrap.
Chill for 30 minutes.
Line a worksurface with parchment paper and lightly flour.
Place the pastry on the paper and roll into a large rectangle about 30 cm x 45 cm.
Pile the filling down the middle of the pastry leaving a 10cm border along each long edge and a 5 cm on along the short edge.
Using a sharp knife make cuts at a 45° angle from the mixture to the edge of the pastry, around 1.5 cm apart.
Starting from the top fold one strip over the filling.
Fold the top strip on the other side over the filling.
Fold the rest of the strips over the filling, alternating sides.
Tuck the ends under.
Brush with Ingredients 8 (beaten egg).
Lift the parchment paper on to a baking tray.
Bake for 45 minutes at 180°C.
Serve.