Put the rhubarb, ginger, half the sugar and orange rind and juice in a medium saucepan. Bring to the boil and simmer gently for 5–6 minutes until the rhubarb is just tender.
Transfer the rhubarb to an ovenproof dish and spoon over dollops of mascarpone.
Place the flour in a bowl. Add the butter and rub in with the fingertips until the mixture resembles fine breadcrumbs. Quickly stir through the remaining sugar, the lemon rind and milk until combined. Place spoonfuls of the mixture over the rhubarb and mascarpone.
Cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 12–15 minutes until golden and bubbling. Serve with low-fat custard.