Lightly dust work surface with flour. Cut the sheet in half and roll each half to 40cm x 20cm. Pop the pastry in the fridge while you prepare the filling.
Sauté the onion, celery and leek in a little olive oil until soft. Allow to cool then mix with the rest of the ingredients until well combined.
Brush along the long edge of each piece of pastry with egg wash. Divide the filling in half and on the opposite side to egg wash spoon the filling longways. Gently roll pastry over so as to roll up with the egg washed side closing the roll underneath.
Place in fridge for 15 minutes.
Cut each log into 12 to 14 pieces. Egg wash liberally with beaten yolk. Snip the top of each sausage roll using scissors by a grind of black pepper.
Line an extra baking tray with baking paper and spread them out over two trays. Bake for 10 minutes at 200C then reduce oven to 180 and continue to cook for a further 10 minutes or until pastry is a dark golden brown.
Served with your favourite sauce or chutney and Rocket leaves.