Combine rhubarb, sugar, stem ginger, and syrup with water in a large saucepan.
Stir and cook for 15-20 minutes until the rhubarb becomes tender.
Allow both the rhubarb and custard to cool. If desired, you can refrigerate them at this point and continue the next day.
Preheat the oven to 180°C.
Whip the egg whites to a soft peak stage, gradually adding the remaining 100g sugar, whipping between each addition until the meringue turns shiny and glossy, forming stiff peaks.
Drain the rhubarb, reserving the syrup, and spoon it over the baked custard.
Swirl the meringue over the top, reaching the edges, and bake for 15-20 minutes until the meringue turns lightly brown.
Serve the dessert hot or warm, accompanied by the reserved syrup.