Preheat the oven to 140°C/gas mark 1. Line a baking sheet with baking paper.
Put the egg whites in a large, clean bowl. Beat together with a hand-held electric whisk for 4-5 minutes until soft peaks form.
Add half the caster sugar and whisk until medium peaks form. Add the remaining sugar and continue to whisk for 2 minutes until you have a smooth, glossy mixture and stiff peaks have formed. Carefully fold in half the ground ginger.
Spoon small rounds of meringue on to the lined baking sheet, leaving space between to allow the meringues to spread slightly. Bake for 1 hour, then remove and let cool on a wire rack. When cool, lightly crush into pieces.
Meanwhile, increase the oven temperature to 200°C/gas mark 6. Put the rhubarb, remaining ground ginger, the juice and zest of half the orange and the honey in a bowl and mix thoroughly. Spread out on a shallow baking tray and roast for 15 minutes until the rhubarb is tender. Allow to cool.
Pour the cream into a large bowl and beat with a hand-held electric whisk until just thickened. Fold in the crushed meringue and the cooled rhubarb. Spoon into serving dishes and top with the orange slices.