Bring the mineral water to a boil and cook the rice with a pinch of salt. Cook over low heat for about 20 minutes. Remove from the heat, pour into a bowl and let cool slightly. Preheat the oven to 180°C (approximately 350°F).
Mix the quark and maple syrup. Mix into the rice. Pour half of the rice into a buttered baking dish.
Rinse the plums, halve, remove the seeds and spread in the baking dish. Cover with the remaining rice. Bake for about 30 minutes. Remove from the oven and serve.