For the pastry: Mix together the flour, salt and caster sugar in a bowl. Then using your hands, rub in the cold butter until it resembles breadcrumbs. Slowly pour in the water and mix until it forms a rough dough. (You may not need all the water). Pour out onto a lightly floured worksurface and knead briefly until a dough forms. Flatten into a disc, wrap in clingfilm and leave to chill in the fridge for 30 minutes.
For the filling: In a medium saucepan add the blueberries, caster sugar and lemon zest and place over a medium-low heat. In a separate bowl combine the lemon juice and cornflour.
Stir the blueberry often until the juices begin to release, then add in the cornflour/lemon mix and increase the heat slightly. Stir often so the mixture doesn’t catch until thickened and jammy but with some blueberries remaining intact. Pour onto a plate or bowl to cool to room temperature.
Take the chilled dough from the fridge, and roll out on a lightly floured work surface until about 3mm thick. Cut into four sections, and line your tart tins, making sure to press the dough into the edges. Then place the tart tins back into the fridge for 20 minutes.
Meanwhile, preheat the oven to 200C/180C Fan/Gas Mark 6 and place a baking tray inside to heat.
For the crumble: Add the softened butter, light muscovado sugar and flour into a bowl and rub together with your hands. You want to leave some lumps of butter intact, so you have a mixture of larger and ‘breadcrumb’ style crumbs. Then set aside.
Take the chilled tart tins out of the fridge, and evenly fill with the cooled blueberry filling and top with the crumble. Carefully remove the hot baking tray from the oven, place the tart tins on to it and return to the oven to bake for 25-30 minutes. If the crumble is starting to brown too quickly – place some foil over the top of each.
Leave to cool in their tins for at least 15 minutes, then carefully remove them. Serve warm.