For the pastry, briefly blend the flour, salt, icing sugar and orange zest in a food processor. Add the butter and whizz to fine breadcrumbs. Mix the egg yolk with the water, add to the mix and briefly whizz again until the mixture sticks together in clumps. Turn out onto a lightly floured surface and bring together, kneading briefly until smooth. Wrap in cling film and chill for 15 minutes, then roll out on a lightly floured surface. Use to line a 2.5cm deep, 23cm loose-bottomed flan tin. Chill for 20 minutes.

Heat the oven to 200°C/fan180°C/ gas 6. Line the pastry case with a crumpled sheet of baking paper and fill with baking beans/rice, then bake for 15 minutes or until the edges are biscuit-coloured. Remove the paper and beans/rice and protect the edges of the case by covering with strips of foil. Return the case to the oven for another 5-7 minutes until the base is golden too. Remove from the oven, discard the foil strips and set aside.

Mix the soured cream, double cream, beaten egg, 6 tbsp of the sugar, the 2 tbsp flour and ground almonds in a large bowl until smooth.

Run a knife around each damson, twist the halves apart and put the neater halves without any stones into one bowl. Prise the stones from the other halves with a small, sharp knife, put them in another bowl and toss with 1 tbsp of the remaining caster sugar. Scatter these sugared damson halves over the base of the pastry case and pour over three- quarters of the filling. Toss the neater damson halves with the rest of the sugar and arrange neatly, cut- side down, on top of the tart in concentric circles. Pour over the rest of the filling.

Bake the tart for 30-35 minutes until the filling has set and is tinged with brown in places. Cool in its tin on a wire rack for up to 30 minutes. Dust with icing sugar, then serve warm (or see tips).