Preheat oven 220C

Cut the sheet of pastry into a 24cm square then round the corners but cutting off excess and place on a baking tray lined with baking paper. (Refreeze excess pastry)

With a sharp knife, score a 22cm diameter circle in the centre being careful not to cut through the pastry. 

Drain the sour cream making sure there is no liquid present. Spread the solid sour cream over the inside of the circle making sure it’s evenly spread.

Cut the pears in half and careful using a water melon baller and knife remove the core. Leaving skin on slice the halves into thin slices making sure you leave the halves all together, not separating the slices.

Fan each sliced half pear onto the centre of the tart working your way around the circle. Each half pear should take up one quarter of the circle when fanned out. Sprinkle the pear with the sugar.

Break the blue cheese into small pieces and sprinkle over the pear evenly.

Take the sides of the pastry and roll them in towards the centre of the tart forming a double outside crust. 

Place the tray into the fridge for 15 mins and allow the pastry to become very cold. Once very cold, remove pastry from the fridge. Add a drop of water to the egg yolk and whisk lightly to form an egg wash. Brush the edges of the pastry with the egg wash.

Place the tray into the centre of the oven and bake for 10 mins. At the 10 minute mark, remove from oven and scattered the chopped walnuts across the top and place back into the oven for a further 10 minutes. 

Once this 10 minutes is over, cover the tart loosely with foil and bake another 10 mins to allow the base to cook completely without burning the pastry top or walnuts.

After the entire 30 mins remove the tray from the oven and allow the tart to cool slightly before moving onto a baking tray to serve.