Cut the rhubarb into 4 cm (1½ inch) lengths and put them in a shallow, non-metallic ovenproof dish.
Put the sugar and water in a small saucepan over a low heat and stir until the sugar has dissolved. Add the vanilla pod and rosewater. Pour the syrup over the rhubarb, cover with foil and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 12—15 minutes until just soft.
Meanwhile, hull and halve the strawberries. When the rhubarb is cooked, discard the vanilla pod and add the strawberries, cover and leave to stand for 5 minutes. Transfer the fruit to serving plates, spoon some of the cooking liquid over each one and add a dollop of mascarpone or yogurt and a sprinkling of chopped pistachios.