Preheat the oven to 180°C/350°F/gas 4. Lightly grease a 25cm loose-bottomed tart tin.
To make the pastry, sift the flour, icing sugar and ginger into a large bowl. Roughly chop and add the margarine and, working quickly, rub into the dry ingredients. Add enough cold water so you can bring the mixture together into a ball. Wrap the dough in clingfilm and chill for 30 minutes.
Once chilled, roll the pastry to ½cm thick and line the tin. Trim any excess and prick all over with a fork. Return to the fridge for 30 minutes.
Bake the pastry blind for 15 minutes. Remove the weight and cook for a further 10 to 15 minutes, until golden brown and cooked through.
For the filling, place the soya milk in a small pan with 200ml water, crush and add the cardamom pods, and the sugar, then warm over a low heat.
Put the cornflour in a small bowl with a few tablespoons of the warm soya mixture and stir till smooth, then return to the pan, stirring to combine, and bring back to the boil. Remove the cardamom pods.
Snap the chocolate into a bowl and pour over the hot soya. Stir to combine. Add the vanilla extract and a pinch of sea salt, pour into the case, chill for 5 to 6 hours.
To cook the rhubarb, cut into 5cm pieces and place in an ovenproof dish with 2 or 3 tablespoons of sugar, depending on how sharp the rhubarb is. Add the cordial, orange zest and juice, and cover with a cartouche (greaseproof-paper disc).
Bake for 20 to 25 minutes, or until soft but holding its shape. Leave to cool. Serve thin slices of the tart with the rhubarb.