Preheat oven to 200ºC (180ºC fan assisted oven)/400ºF/Gas 6.
Combine prepared rhubarb with 75g/3oz caster sugar and place in an ovenproof dish, covered. Roll out pastry large enough and line a 23 cm/9" flan tin.
Place pastry in oven and bake blind for approx. 18 minutes, removing beans and paper for final 5-7 minutes. At the same time as cooking pastry case, place rhubarb in the oven on the other shelf, stirring once or twice and cook until just al dente. Once both finished, reduce oven temp. to 160ºC (140ºC fan assisted oven)/325ºF/Gas 3.
Once rhubarb is cooked drain off juices (keep these to serve with flan) and scatter approx. one third of fruit over cooked flan base.
Meanwhile whisk eggs and sugar together until pale, whisk in the finely grated rind and juice of the oranges. In a separate bowl lightly whip the cream until just holding in peaks, then gently whisk into the egg mixture.
Gently pour in the egg mixture into flan and return to the oven to bake for 35-40 minutes until 'custard' is just set. Remove and cool.
Serve after scattering the remaining rhubarb over the top and dredge with icing sugar.