For the pastry, place the plain flour, unsalted butter, icing sugar, eggs and a splash of cold water in the TM bowl. Mix 20 sec/speed 6.
Remove the pastry and place on a floured surface. Roll out to fit a 23cm tart dish. Line the tart dish and trimm off any excess pastry.
Prick the base with a fork and refrigerate for 30 minutes. Meanwhile, preheat the oven to 150°C.
Chop the rhubarb in 5cm pieces and place on a large rectangular baking tray lined with greaseproof paper. Sprinkle with 200g caster sugar and bake in the oven for 25 minutes. Remove and place aside.
Fill the pie base with greaseproof paper and baking beans. Increase the oven temperature to 190°C. Blind bake the pie case for 15 minutes, then remove the baking beans and greaseproof paper and bake again for 10 minutes.
To make the meringue topping, place 4 egg whites into TM bowl. Whisk 4 Min/Butterfly/speed 3. While whisking, slowly add in 200g caster sugar and 2 tsp cornflour through the lid in a thin stream. Remove and place in a separate bowl. Repeat the same process again.
To assemble the pie, fill the baked shell with the rhubarb and top off with the meringue. Bake in the oven for 30 minutes. Leave to cool, then slice up.