Start with the onions. Heat the butter in a large pan over medium-high heat and add the onions. Cook, stirring often, until they wilt. Sprinkle salt over them. Lower the heat to medium and cook the onions down, stirring occasionally, until they are very soft and brown. Add some marjoram as it cooks. This process should take between 20 and 40 minutes, and you may need to cover the pan to keep the onions from sticking to it. When they're done. turn off the heat.
Meanwhile, boil the potatoes in salty water until tender. Drain and let them steam a minute or two, then push them through a ricer or a food mill. If you don't have either, just mash them well.
Get a large pot of water going so you can boil the shupfnnudeln. You can use the potato water but you'll need to add to it. Melt 3 tablespoons butter in a large pan over low heat.
Let the potatoes cool for a few minutes, then quickly mix in the egg, white pepper, nutmeg and flour. Knead into a soft, but smooth dough.
Flour a work surface. Cut the dough into 4 pieces. Form a log with one piece about 1 inch thick. Use a knife or bench scraper to cut that log into 1-inch pieces.
Using your palms with a 1-2-3 back-and-forth motion, roll a piece into a spindle shape. Set it down on the floured surface and continue with all the rest of the dough.
When the shupfnudeln are all done, gather up about a quarter of them and boil in the salty water. When they start to bob to the surface, turn the heat to high on the pan with the butter. Gather up the dumplings and let them drain a second or two, then put them in the pan with the butter, shaking the pan as you do.
Brown the shupfnudeln in the butter, turning them as needed. This sets them and gives you some nice Maillard reaction. Move the finished dumplings to a tray or plate. Repeat the process with the remaining dumplings. You will likely need more butter.
To finish, brown the sausages in the pan you cooked the dumplings. Chances are they'll still be uncooked at the center, so drop them into the water you boiled the dumplings in and let them cook there for 5 to 10 minutes. Remove them and slice.
Heat the onions back up and add the sausage, sauerkraut, parsley and shupfnudeln. Mix well and serve. Black pepper is a nice addition.