Bring milk and cream to the boil in a saucepan. Stir in rice pudding, bring to the boil and let it swell at low heat for 30-40 minutes, stirring from time to time.
In the meantime, clean and wash the rhubarb and cut it into pieces of about 1.5 cm width. Put 100 ml maple syrup into a pot and simmer on low heat for 5-6 minutes until the rhubarb is soft but not yet decomposed.
Stir pudding powder and 3 tablespoons of water until smooth. Stir into the boiling rhubarb and simmer for 1 minute. Put the rhubarb into an ovenproof dish.
Preheat oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer). Stir 75 ml maple syrup into the rice pudding. Separate the egg. Beat egg white and 1 pinch of salt until stiff. Whisk egg yolk and stir into the rice pudding.
Fold in the beaten egg white.
Spread rice pudding on the rhubarb, sprinkle with oat flakes. Sprinkle with approx. 1 tablespoon maple syrup. Bake in a hot oven for about 10 minutes.