Place the first round of puff pastry (pâte feuilletée) on a parchment or nonstick baking tray.
Sprinkle the crumbled cheese and chopped walnuts over the top, leavinga bit of room at the edge of the dough.
Now place the second circle of pastry on top and press down lightly. Do not squeeze the edges together.
Whisk the egg yolk with 2 teaspoons of water.
Brush the egg mixture over the top of the pastry.
Put the tart back in the fridge for about 10 minutes.
Preheat the oven to 200C (400F).
Remove the tart from the fridge and place a glass downwards in the centre - to mark the starting point.
Slice the dough, stopping at the glass, into quarters, then eighths and finally 16ths. You will end up with 16 "rays."
Remove the glass, and gently lift the outside of each ray and twist the strand twice. Carefully turning each ray, but try not to squeeze the dough.
Brush the unglazed bits of pastry with the egg yolk mixture.
Return to the fridge for 10 minutes.
Bake until puffed and golden brown 20-30 minutes depending on your oven's temperature.