For the pastry: Mix the flour, caster sugar and salt together into a bowl. Add the diced butter, rubbing in with your hands until it resembles breadcrumbs. Add the water a little at a time until the dough starts to come together (you may not need to use all of it). Knead lightly in the bowl, until it comes together to form a uniform dough. Try not to handle it too much. Press the dough into a disc, wrap in clingfilm and put in the fridge for 30 minutes.
Take the chilled dough from the fridge, and roll out onto a lightly floured work surface until about 3mm thick. Line a rectangular tart tin with the pastry, pressing the pastry into the edges and trimming off any excess. Then place the tart into the fridge to chill for another 30 minutes. Pre-heat the oven to 180C/160C Fan/Gas Mark 4.
For the frangipane: Meanwhile, beat the sugar and butter together in a medium bowl using a wooden spoon or silicone spatula until light and fluffy. Add the eggs one at a time, beating until fully combined before adding the next. Finally, add the ground almonds and mix to combine.
Then wash your peaches, destone and slice thinly. Set aside.
Once the pastry is firm, line it with baking paper and fill with baking beans. Then blind bake for 20 minutes. Then remove the beans and baking paper and bake for another 5 minutes until the pastry is dry. Spread the frangipane filling into the baked pastry case, and fan the peach slices on top in different directions.
Bake in the oven for 25-28 minutes until nicely golden brown. When the tart has a few minutes left in the oven, heat the jam either in a small saucepan or in the microwave until loosened. Then once the tart has been removed from the oven, brush the warm jam over using a pastry brush, to glaze the tart.
Leave the tart in the tin to cool before slicing and serving.