Mix all the ingredients for the dough in a food processor or by hand until you have a cohesive dough. Wrap in cling film and leave to rest in the fridge for at least half an hour.
Meanwhile, make the filling. Heat the cream to a luke warm temperature and set aside. Melt the sugar in 1 tbsp of water and allow to caramelize gently (do not stir as the sugar will lump). When you have a nice golden brown caramel, slowly add the cream while stirring (be careful not to splash). Bring to the boil and simmer briefly. Finally stir in the chopped walnuts.
Preheat the oven to 180˚C (350˚F)
Roll out 2/3 of the pastry into a 4mm thick disc and line a greased baking tin. Spoon the filling into the tin and cover with the rest of the pastry. Use a knife to make a few indentations in the dough to allow the steam to escape and prevent the dough from bulging. Brush the top of the cake with the egg wash.
Bake at 180˚C (350˚F) for about 40 minutes until golden brown.
Leave to cool and serve with a nice dollop of whipped cream.