Grease a 30cm/12in loose-bottomed tart tin. To make the pastry, click here for the recipe. Turn the gluten free dough onto a floured (gluten free) board, knead gently into a smooth ball then roll the pastry out and use it to line the tart tin. Prick the base with a fork, line with foil and chill in the fridge for 30 minutes.
Meanwhile, make the filling. Preheat the oven to to 150C/300F/Gas 2. Cut the rhubarb into 5cm/2in lengths and place in an ovenproof dish. Sprinkle with the sugar and orange zest then cook, uncovered, in the oven for 20 - 25 minutes or until the rhubarb is just tender but still holding its shape. Increase oven to 190C/375F/Gas 5.
Add baking beans to the tart case and bake blind for 15 minutes, then remove the foil and the beans and return the tin to the oven for about eight minutes or until pale golden-brown. Remove the tin from the oven and increase the oven to 200C/400F/Gas 6. For the meringue, place the egg whites in a large bowl and whisk until they form soft peaks. Gradually add half the sugar, whisking all the time, then fold in the cornflour and remaining sugar and whisk until the mixture is smooth and glossy.
To assemble the pie, fill the tart case with the rhubarb, standing the pieces on end. Pile most of the meringue on top of the pie, keeping enough back to then pipe pointy peaks over the surface. Bake in the oven for about 30 minutes, or until the top is crispy and very pale gold. Allow the pie to cool before removing it from the tin.