In a microwave-safe bowl, sauté the onion with a tablespoon of butter for 2-3 minutes and add to the bread rolls. Beat the egg with the milk and pour over the mixture. Stir in well and let it steep for about 30 minutes, stirring frequently.
Prepare 6 coffee mugs. Melt the remaining butter in one of the coffee cups and then brush all the cups with the melted butter. Shape 6 dumplings of equal size with moistened hands and distribute them on the cups. Arrange in a circle in the microwave and cover with microwave foil or hood. Cook at full power for 6 minutes and serve.