Preheat the oven to 190°C/170° fan/375°F/gas 5. Cut the rhubarb into 4cm lengths and place in a shallow baking dish with 75g caster sugar and 4 tablespoons water. Bake for 20 minutes, then drain and cool. Reserve the juice.
Lower the oven temperature to 180°C/160°C/350°F/gas 4, lightly butter a 2-litre dish and arrange the rhubarbover the base. Halve the passion fruit and scoop out the pulp of the passion fruit and pass through a sieve set over a bowl, then discard the seeds.
Whisk the egg yolks and remaining caster sugar in a bowl until thick and pale, then whisk in lemon zest and passion fruit juice. Mix in the flour and a pinch of salt, then stir in the milk and yogurt until combined.
In another bowl, whisk the egg whites and cream of tartar together to form soft peaks. Fold the egg whites gently into the passion fruit mixture.
Pour the mixture over the rhubarb and place the dish in a large roasting tin. Pour boiling water into the roasting tin (bain marie) until it comes halfway up the sides of the dish and bake for 40-45 minutes, or until the centre of the pudding is firm to the touch. Serve warm or cold with the rhubarb cooking juices, and dust with icing sugar.