For the jam:
Put the apricots and the lemon juice into a pan on a medium heat and cook for around 30 minutes (keep the lid on if your apricots are quite hard as it will help them soften), stirring them regularly and squashing them with a wooden spoon to help release their juices.
When the fruit has collapsed, take it off the heat and pass it through a sieve, with a bowl underneath, to remove the skins and any tough fibers. You should be left with approx. 650g of apricot purée.
Put this back in the pan with the sugar and pinch of salt and cook on low-medium for another 40-50 minutes until it has thickened. Watch it and stir it regularly to make sure it isn’t burning on the bottom. The jam will continue to thicken as it cools but this will leave you with a softer set jam that is just what we want for the crostata. You’ll need around 400-450g for the crostata so any leftovers can be kept in a jar in the fridge for toast, yoghurt, porridge or biscuits.
For the pastry:
Put the flour, butter and sugar into a mixing bowl and use your fingers to rub everything together until no large chunks of butter remain and you’re left with a loose crumb.
Add the lemon zest, vanilla and a pinch of salt, stir to combine and then make a well in the middle with your fingers. Crack one whole egg and one yolk (reserve the extra white for brushing) into the well and start mixing everything together.
Turn the dough onto a clean work surface and knead lightly, just until you have a nice, smooth ball. Try not to handle it too much. Wrap the dough in cling film and let it rest in the fridge for a minimum of 30 minutes or, even better, overnight.