Heat oven to 190C/170C fan/gas 5.
Put the flour in a food processor with
the butter and sugar, and mix until it
forms fine breadcrumbs. Add the egg
yolk and 1-2 tbsp cold water, and pulse
to make a firm dough.
Have ready a 24cm tart tin. Briefly
knead the pastry on a lightly floured
surface, then roll out to a round about
5cm larger than your tin. Lift onto the tin
with the help of your rolling pin, then
press into the corners using your finger.
Do not trim the pastry. Fill the pastry
case with a round of baking parchment
and baking beans. Chill for 10 mins. Blindbake
for 10 mins, remove the paper and
beans, then bake for 5 mins more.
To make the filling, put the sugar in
a large pan with 3 tbsp cold water. Heat
gently, stirring to dissolve the sugar. When
the sugar is completely dissolved, increase
heat and bubble until the syrup has turned
a rich caramel colour. Remove from the
heat and stir in the butter until it has
dissolved, then stir in the cream. Return
to the heat and boil hard, stirring until the
sauce is thick enough to leave a gap on
the base of the pan when you draw your
spoon across it. Stir in the walnuts.
Fill the pastry case with the nut mixture,
levelling it with a fork. Return to the oven
for 8-10 mins until the filling is bubbling.
Cool for 10 mins before removing
carefully from the tin. Serve warm or cold.