For the pastry, sieve the flour into a mixing bowl. Add the butter, cooking fat, Parmesan and thyme and rub into the flour until the mixture resembles breadcrumbs.
Add the egg yolk and the water and mix to form a firm dough. Wrap the dough in cling film and place in the fridge for 30 minutes.
On a floured surface, roll out the pastry to fit one large flan tin. Line the tin with the pastry and prick the base all over with a fork. Place in the oven and bake for 15 minutes.
Place all the filling ingredients, except 50g of the cheese, into a bowl and gently combine. Brush the base of the pastry with the egg yolk and pour the mixture into the flan case, sprinkle on the remaining cheese and bake for 10-15 minutes until firm to touch.