The Recipe

Peel and cut the onions to small dice.

Place in a pan with a little splash of water and cover with a lid - put on a low heat to cook gently - the onions are to be cooked through without browning, so keep an eye on them, stirring occasionally and keeping the heat low so as to simmer them slowly.

Put the fat and flour in a food processor and run it until the mixture resembles fine crumbs.

(if you don't have a food processor, you can rub the fat and flour together with your fingertips - it just takes longer).

Add the egg yolk and just a little water, then run the food processor again - the mixture should start to clump into pieces of soft dough.

Turn the mixture out onto a clean floured surface and squeeze it together into a ball.

Divide it, cutting off about one third - the larger portion will form the base and the smaller one will form the lid.

Roll out the larger piece of pastry until it's big enough to line a Swiss roll tin.

Dust the board and the top of the dough before, during and after rolling, to prevent it sticking.

Now is a good time to preheat the oven - 180 C.

Roll the pastry sheet up loosely onto the rolling pin to transfer it, then unroll it into place in the tin, then trim around the edge so there is a little bit sticking up.

Don't worry if it seems a little ragged - this often happens - if there are any cracks or gaps, just moisten and patch them with a spare trimming of dough.

Roll out the smaller piece of dough big enough to cover the tin, then set it aside, ready to go on top.

By this time, the onions should be cooked - they should be translucent and cooked, but not browned at all, and they should be moist, but with no actual liquid left in the bottom of the pan.

Add the mustard, then grate and add most of the cheese (set aside a small handful of grated cheese to sprinkle on top later) - stir the onion, mustard and cheese together.

The cheese - not shown in this photo - will melt and form a sauce coating the onion pieces.

Spoon the filling into the pastry shell, spreading it out carefully. If the filling mix is still hot, you will now need to work fast to prevent the pastry at the bottom from going soggy.

Place the pastry on top of the filling, then trim around with a knife and finally pinch all the way along the edge to seal.

Brush the top crust with milk, then sprinkle on a little grated cheese.

Place in the oven and bake for 25 minutes, or until light golden in colour.

Allow to cool for at least ten minutes before cutting into squares or fingers and removing from the tin.