Roll out the pastry and use to line a deep 23cm/9in flan tin. Chop the rhubarb into 3cm/1in lengths and tip into a large bowl with the flour and sugar. Mix well, then tip into the pastry case, spreading the rhubarb to level it.
To make the topping, mix together the flour, cinnamon, sugar and nuts. Add the butter, cut into small pieces, and rub in with your fingertips until the mixture resembles coarse breadcrumbs. Sprinkle evenly over the top of the rhubarb. Set aside until ready to bake, up to 6 hours.
Preheat the oven to 190C/Gas 5/fan oven 170C. Bake the pie for 40-45 minutes until the topping is crisp and golden and the rhubarb is tender – test by spearing a piece of fruit through the topping with a knife. Serve warm or cold with single cream or vanilla ice cream.