To make the pastry, put the flour, salt and butter in a food processor and process for 5 seconds. The butter should still be visible and in small pieces. Add the water and process for 5 seconds more – just enough time for the dough to start holding together. Little pieces of butter should still be visible throughout (this is important to achieve a delicate, flaky pastry).
Remove from the machine and lightly gather into a dough with your hands. Form into a ball, flatten slightly and wrap in cling film. Rest in the fridge for 30 minutes.
Preheat the oven to 200°C/Gas 6. Roll the dough out on a lightly floured cool surface to a large, thin round, about 30cm in diameter. Lift the dough onto a baking sheet.
For the almond layer, toss together the ground almonds, flour and sugar and scatter over the pastry.
Peel, quarter and core the apples, then slice into 3mm wedges. Place in a bowl and sprinkle with the salt and all but 1 tbsp of the sugar. Toss well to combine. Taste and add a little more salt if the apple seems too sweet.
Arrange the apples evenly over the pastry, leaving a 5cm clear border. Fold the edges of the pastry up over the fruit, crimping and tucking them gently as you do so. Sprinkle the remaining sugar on top, scatter over the walnuts and dot with the butter.
Bake in the oven for 40–45 minutes, or until the pastry is crisp and golden brown. For the best possible flavour and fragrance, eat within an hour.