Preheat the oven to gas 4, 180˚C, 160˚C fan. Toss the rhubarb with 75g (3oz) of the sugar then spread over a baking tray. Sprinkle with tbsp orange juice, roast for 20 minutes, then cool.
Roll out the pastry to line a 23cm (9in) loose-bottomed tart tin. Leave excess hanging over and prick the base with a fork. Chill for 20 minutes until firm. Line with baking paper and baking beans and bake for 15 minutes. Remove the beans and paper, then bake for a further 10 minutes. Trim the edges when cool.
Remove the rhubarb from the syrup. Mash half and spread over the pastry. Beat together the remaining sugar, butter, almonds, eggs, almond extract, baking powder, salt and flour until creamy. Spoon the mixture over the rhubarb purée, then scatter with the remaining rhubarb, flaked almonds and 1 tbsp sugar.
Bake for 45 minutes, or until golden and risen. Cool in the tin until warm. Stir the orange zest through the custard, reserving some to sprinkle on top, and warm. Slice the tart and serve with a drizzle of the rhubarb syrup and the zesty custard.
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