Wash and chop the chard and cook in a saucepan with water and salt. After approx. 10 minutes, drain the chard and collect the brew. Let the butter get hot in the pan, then stir the flour quickly with a whisk and deglaze with milk. Add the brew to taste.
Season with salt, pepper and nutmeg. Finally stir in the mascarpone, bring to the boil briefly and pour over the chard.