For the napkin dumplings, cut the bread into fine cubes. Fry the onion and bread cubes in butter until golden brown. Allow to cool slightly.
Stir milk and eggs together, season with salt and add to bread cubes. Let the liquid soak in. Mash with a fork. Dust the flour over the top, also incorporating it.
Wet a napkin, butter the center. Place the bread mixture, formed into an oblong dumpling with wet hands, on the napkin, forming it into a roll.
Tie off with a thread on the right and left and tie the ends of the thread, tied into loops, to a wooden spoon. Place this diagonally over a pot filled with boiling hot salted water.
Let steep for 40 minutes.
Roll the napkin dumpling out of the napkin and cut diagonally into 1 cm thick slices.