Rich Shortcrust Pastry

I made this other day for the onion tart, using one of nigella's simpler recipes.

- Put 120g flour in a processor together with 60g cold butter, cubed.  Nigella recommends putting it in the freezer for 10mins as well, but i have so far ignored this.
- Process until mixture resembles oatmeal.
- Beat an egg yolk together with 1 teaspoon of yoghurt or creme fraiche or orange/lemon juice.
- Add the beaten egg to the flour and process until the mixture is just about to clump together.
- If it doesn't, add tiny amounts of iced water, until it does.
- Remove from the processor, cover in clingfilm and keep in the fridge until needed.

If you want ordinary shortcrust pastry, then ignore the egg, and just use iced water to bind it.