Add the onion and herb mixture and the whipped egg yolk and butter mixture and mix in vigorously. Add the egg white, which has been beaten to stiff snow, and carefully add to the mixture with a wooden spoon. Spread out a damp cloth or napkin (approx. 80 x 80 cm) and place the dumpling mixture on top with damp hands or a spoon in the form of a thick sausage. Roll the mixture loosely in the cloth as it expands as it cooks. Tie off the cloth at the ends with kitchen thread. Put in boiling salted water. Cook with the lid slightly open for about 1 hour. Turn halfway through the cooking time.
Wrap the finished dumpling out of the cloth and cut into slices.