Preheat the oven to 200C/400F/Gas 6.
In a food processor, blend the flour and butter together until the mixture resembles breadcrumbs.
Add the sugar and salt to the food processor and pulse again until combined.
In a small bowl, mix the egg with the cold water and gradually add enough liquid to the flour mixture to form a soft but not sticky dough.
Place the dough onto a clean, lightly-floured surface and briefly knead the dough until smooth.
With a clean, lightly-floured rolling pin, roll the dough into a rectangle shape 0.5cm/¼in thick and trim the edges.
Lay the pastry on a non-stick baking tray and sprinkle over the nuts, honey, rhubarb and caster sugar, leaving a border of pastry 1cm/½in wide around the edges.
Bake in the oven for 12-15 minutes, or until the pastry is crisp and golden-brown.
To serve, arrange the orange segments in a row down the centre of the tart and drizzle with the double cream.