To make the pastry, sift the flour into a bowl with a pinch of salt, sugar and ground ginger. Rub in the butter until you have a breadcrumb consistency, then add 30-50ml of cold water, stirring with a flat-bladed knife until the crumbs form a dough. Knead the pastry briefly until smooth, then wrap in cling film and refrigerate for 30 mins.
Heat oven to 200C/180C fan/gas 6. Line a baking tray with baking parchment. Arrange the rhubarb in the tray in a neat layer, scatter over 250g of the sugar and scrape out the vanilla pod over it. Add the orange juice and zest and cover tightly with foil. Bake for 20-30 mins, until the rhubarb is soft. Remove from the oven and cool. Strain off the syrup using a sieve – you’ll need this later for the meringue.
Roll out the pastry on a floured surface. Line a 20cm tart tin, leaving an overhang. Pop into the fridge to chill for 30 mins. Once chilled, cover the case with baking parchment, fill with baking beans and blind bake for 25 mins, until golden. Remove the baking beans and bake for 5 mins more to crisp up the bottom. Allow to cool, then remove from the tin to a plate.
For the custard, combine the remaining sugar with the egg yolks and whisk vigorously, until really pale and frothy. Gently heat the cream and milk in a pan until it’s almost boiling. Remove from the heat and briefly allow to cool before pouring a little of the cream mix onto the egg yolks and stirring to combine. Pour the egg yolk mix back into the pan and heat gently, stirring with a wooden spoon. Cook until the mixture coats the back of a spoon (about 5 mins). Remove from the heat and pour into a jug or mixing bowl.
Divide the rhubarb in half, reserving the pinkest, prettiest lengths. Blitz the other half to a purée in a food processor and add to the custard. Fill the tart and top with the rhubarb. Chill until needed.
To make the Italian meringue, measure out 160g of the rhubarb poaching syrup and bring to the boil. You need to it be about 118C (this will cook the egg whites to set the meringue). In the bowl of a stand mixer, whisk the egg whites until they form soft peaks. Add the cream of tartar and mix. When the syrup has reached 118C on a cooking thermometer, carefully pour it down the sides of the stand mixer and whisk until you have a smooth, fluffy meringue. Fill a piping bag with a medium star nozzle and pipe little meringue kisses over the tart. Use a blowtorch to gently scorch the meringue and serve straight away.