Prepare and cook the rhubarb at least a day ahead of serving the sorbet. Wipe and trim the fruit, then slice it quite thinly into a flameproof casserole. Sprinke the finely grated orange zest over it and drizzle on 2 tablespoons of the honey. Cover and leave in a cool place for 2 hours or more until the sweetener has drawn out some of the rhubarb juices; this liquid will prevent sticking during cooking. Cook the rhubarb in the covered dish until perfectly tender I think this is best done in a low oven but you can use a very gentle flame on top of the stove if you prefer. Let the cooked rhubarb cool slightly, then add the remaining 2 tablespoons honey and whizz to a very smooth puree in a food-processor. When the puree is quite cold, freeze it in a chilled loaf tin, or other suitable container, until firm round the edges. Beat the half-frozen sorbet until slushly. Whisk the egg whites and fold them in so the mixture looks like a pale pink snow. Cover and freeze until solid. Beat to break up ice crystals and to increase bulk, then freeze the sorbet again until you are ready to serve it. Shortbread fingers make a good crunchy accompaniment.