1. Start by making the pastry. Put the fl our in a large bowl and rub the butter in with your fingertips until it resembles fine breadcrumbs. (Alternatively, pulse together in a food processor.) Stir in the sugar, then mix in 4 tbsp cold water until it just comes together, adding an extra 1-2 tbsp water, if needed. Knead briefly until smooth, flatten into a puck and wrap in cling film; chill for 15 minutes.
2. Meanwhile, steam the pumpkin (or squash) for 25-30 minutes, until very tender; if you don’t have a steamer, use a colander set over a pan of boiling water, covered. Set aside to cool slightly.
3. Preheat the oven to 200°C, gas mark 6. On a lightly fl oured surface, roll the pastry to the thickness of a £1 coin and use to line a 3cm-deep, 22cm loose- bottomed tart tin. Cut off the excess pastry and push up the sides so they’re just higher than the side of the tin.
4. Line the case with baking parchment, fill with baking beans and place on a baking sheet. Blind bake for 15 minutes, until golden on the edges. Remove the parchment and baking beans and brush the pastry base with a little of the beaten egg. Return to the oven for 10–12 minutes, or until the pastry is golden and crisp. Set aside to cool.
5. Whizz the pumpkin (or squash) in a food processor (or push through a sieve) to make a smooth purée. Transfer to a large bowl and beat in the sugar, 2 mixed spice and the salt, followed by the evaporated milk and remaining egg, until combined. Pour into the cooled tart case; bake for 15 minutes.
6. Reduce the heat to 170°C, gas mark 3, and bake for 25 minutes, until set with just a slight wobble in the centre. Set aside for 5 minutes, then remove from the tin and cool on a wire rack. Mix the icing sugar with the remaining ½ tsp mixed spice and dust over the pie. Serve warm, with whipped cream, if liked.