Heat oven to 220C/200C fan/gas 7.
Heat the olive oil in a large frying pan,
then add the butter. Once sizzling, add
the leeks and cook over a medium heat
until softened but not coloured. Stir in
the thyme and half the lemon zest, then
increase the heat. Add the lemon juice
and cook for about 30 secs until the
lemon juice reduces, then season well.
Remove from the heat and cool slightly.
Unroll the pastry and lay on a baking
sheet lined with baking parchment.
Lightly mark a 1cm border around the
edges with the tip of a sharp knife, then
prick the base all over with a fork.
Spread the lemony leeks on top of the
pastry, within the border. Crumble over
the cheese, scatter with the walnuts,
then season with pepper. Drizzle with
some olive oil, brushing the edges with
a little oil as well. Put tart in the oven
for 15-20 mins until the pastry puffs up
around the edges and is golden brown.
Scatter with parsley and the remaining
lemon zest. Serve hot, warm or cool.