Add the plain flour to a medium sized bowl and mix with a pinch of salt if using unsalted butter.
Cube the butter, then add to the bowl with flour and rub the butter into the flour until it looks like coarse breadcrumbs. Try to work as quickly as you can, to ensure that the butter stays as cold as possible. You can rub the butter into the flour by rubbing the butter inbetween your fingers. You can also use a dough cutter to cut the butter into smaller and smaller pieces until it resembles coarse breadcrumbs. If you have access to a food processor, you can use it to rub in the butter quickly and easily. Simply add the flour and salt to the food processor and add the cubed butter to the food processor. Then blitz the mixture until it resembles coarse breadcrumbs.
Add the sourdough discard to the flour and butter mixture, then mix until it comes together. You can lightly knead it for a second or two but try not to over-knead or mix or it will result in a chewy crust.
If you find the dough is a bit too sticky add a sprinkling of flour. If you find it is a bit too dry, you can add a little bit of extra sourdough discard or a drop of water. It should end up being a nice smooth dough.
Leave it covered in the fridge, either wrapped in a beeswax wrap or in a closed container. This will prevent any dehydration of the shortcrust dough, ensuring it stays smooth and supple.
Storing & blind baking
If you want to blind bake your shortcrust pastry, a good rule of thumb is to bake it at 200C or 390F. Cover it with baking parchment and fill it with dried beans or baking weights. Then bake for 10 to 15 minutes covered and another 5 minutes uncovered. Or just follow your recipe's instructions.
If you want to store it for longer, you can store it covered in the fridge for up to 3 days. Although personally, I have stored mine for longer than that. Since the lactic acid bacteria in the sourdough shortcrust pastry keep it fresher for longer.
Notes
Tips:
When you go to grease and flour your pie or tart dish, first use a pastry brush to brush some olive oil or butter on your pie dish, then dust it with flour. This will create a very good non-stick layer.Leaving your dough to rest will prevent your dough from shrinking during the baking time.Keep your work and kneading to a minimum for light and crumbly shortcrust pastry.Keep your ingredients and pastry as cold as possible since that will ensure that your crust will be as flaky and crispy as possible with all those delicious tiny buttery layers.
Making sweet, plain or savoury shortcrust pastry
This recipe is a wonderful all-round recipe, that can be used for sweet and savoury recipes alike. But in case you prefer a shortcrust pastry crust that has a hint of sweetness or a savoury bite to it, you can alter the recipe slightly to suit your taste.If you want a slightly sweetened sourdough shortcrust pastry crust you can add a tablespoon of honey or demerara sugar. You can also opt to add an egg yolk to your crust for some added richness and a fuller flavour which works well in sweetened shortcrust. A sweetened shortcrust pastry is especially delicious when paired with some fruit. Especially when that fruit might be a bit less sweet or even a bit tart. In which case, the sweetness of the pastry balances out the tartness of the fruit.When making a savoury dish. It is sometimes nice when you can combine the flavours of the filling with the shortcrust base. For example, when you're making a chicken pot pie, with sourdough shortcrust pastry, and you add some thyme to the shortcrust pastry. You can essentially add whatever herbs or spices you prefer to your shortcrust pastry. Although when you add them will make a difference in the final pastry. Spices are best mixed into the plain flour before you add anything else. Whereas herbs I find are best added after the butter has been rubbed in, the same goes for cheese or small bacon bits.