For the filling: (can make the day ahead)

1. Mix the rhubarb with 70 g (2.4 oz) of the sugar and the orange zest and juice. Scatter in a roasting tray and roast at 200°C (400°F) for 20 minutes until soft.
2. Purée the rhubarb and its juices in a food processor, then pass through a sieve into a saucepan, pushing through with a wooden spoon to extract as much as possible.
3. In a small bowl, mix the remaining sugar with the cornflour and enough of the rhubarb purée to make a paste. Return to the pan with the rest of the purée. Bring to the boil and bubble for 1 minute to activate the cornflour. Taste — if too tart, add 1 tbsp more sugar.
4. Mix the egg yolks with the melted butter, then stir into the rhubarb mixture to make a curd. Scrape into a bowl, cover, cool, and chill.

For the coconut pastry: (can make the day ahead)

1. Put the desiccated coconut into a food processor with 1 tbsp of the flour and pulse to a fine mixture. Transfer to a large bowl and mix with the remaining flour. Stir in the icing sugar.
2. Add the cold cubed butter and, using your fingertips (or a pastry cutter), rub it into the flour mixture until the mixture resembles fine breadcrumbs.
3. Add just enough iced water to bind. Knead briefly until smooth, wrap in cling film, and chill for 20 minutes (or refrigerate until needed).

To assemble the tart: (can be done the day ahead)

1. Preheat oven to 200°C (400°F).
2. Roll out the pastry on a lightly floured surface — the dough is a bit tricky to work with and easily sticks; use a sheet of baking paper over the top of the dough when rolling if needed. Use the pastry to line a 20 cm × 3.5 cm (8 × 1.3 inch) fluted loose-bottomed tart tin. Patch any cracks with extra dough. Chill for 30 minutes. (You can allow the dough to drape over the sides and trim the edges with a sharp paring knife once fully baked and still warm.)
3. Line the pastry case with baking paper and baking beans or rice. Blind-bake on a baking sheet for 12 minutes, then remove the paper and weights and bake for another 5 minutes until crisp and golden. Set aside.
4. Turn the oven down to 190°C (375°F). Pour the rhubarb curd into the pastry case and bake for 30 minutes until set. Cool completely, then chill.

For the meringue topping: (do on the day of serving)

1. Preheat oven to 190°C (375°F). Bring the chilled tart to room temperature.
2. Whisk the egg whites until stiff, add 1 tbsp of the sugar, then whisk until stiff again. Continue adding the remaining sugar a tablespoon at a time, whisking until glossy and stiff. Fold in the desiccated coconut and spoon the meringue over the rhubarb curd, forming pretty peaks.
3. Bake for 15 minutes until the meringue is golden and crisp on the outside. Remove, cool, and serve.