How to Make It
Preheat the oven to 190°C/375°F/Gas Mark 5, placing an oven rack in the lower part of it. Butter a 25-cm/10-inch loose-bottom tart pan.
Roll out the pastry (dough) thinly on a lightly floured work surface and use to line the prepared pan.
If using spring rhubarb, arrange it in the pastry case (shell) and sprinkle with the demerara (turbinado) sugar. If not using spring rhubarb, put the caster (superfine) sugar and 200 ml/7 fl oz (3/4 cup) water into a pan and stir together over medium heat until the sugar dissolves. Bring to a gentle boil, then add the chopped rhubarb and simmer for 1 minute – you don’t want it to lose its shape. Drain the rhubarb, then arrange inside the pastry case.
Place the filled case on a baking sheet and bake on a low oven rack for 40 minutes. Take the tart out of the oven and carefully slip off the outer ring, leaving the tart sitting on the base. Return to the oven on the baking sheet for another 5–10 minutes so that the sides get really crisp.
Slide the tart onto a plate and sprinkle with caster sugar, if desired. Let cool to room temperature before serving.