Bring 2 l water and broth to the boil. Covered Maultaschen and cook for 10-12 minutes.
Clean, wash and drain the lamb's lettuce. Peel onions, cut into rings. Mix vinegar, mustard, salt, pepper and sugar. Fold in the oil.
Drain the Maultaschen, cut into strips, mix with half the vinaigrette. Mix the rest with salad and onions. Arrange Maultaschen on the salad.