Place rhubarb into a pan with the water and the orange juice and zest and the caster sugar, cover and stew slowly until tender.
Place into a food processor until a fine paste is achieved, allow to cool then add the double cream and yoghurt.
Spoon into a ramekin (or you could use a teacup) and allow to set.
Mix the orange zest with the crème fraiche and serve a spoonful on top of the mousse.
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