Preheat the oven to 220°C/fan200°C/gas 7. Scatter the rhubarb in 1 layer on a baking tray. Dot with the butter, drizzle with the orange juice, sprinkle over the sugar and zest and roast for 15 minutes, until just tender. Set aside until cool.
Divide between 6 glasses or dishes. In a small bowl, roughly stir the lemon curd into the mascarpone with a kitchen knife to form a ripple effect and spoon on top of the rhubarb. Scatter over the biscuit crumbs and serve dusted with icing sugar.