Cook half of the potatoes.Peel 1 pound of the potatoes and cut them into chunks. Place them in a pot of salted water and boil until tender. Drain well, mash while warm, and let cool slightly.
Grate the raw potatoes.Peel the remaining 1 pound of potatoes and grate them finely or medium-fine. Place the grated potatoes in a clean kitchen towel or cheesecloth and squeeze out as much liquid as possible.
Save the potato starch.Let the squeezed potato liquid sit for a few minutes. Carefully pour off the liquid and keep the white starch that settles at the bottom of the bowl. Add this natural potato starch back to the dumpling mixture.
Prepare the beef filling.Warm the butter or oil in a skillet over medium heat. Add the chopped onion and cook until soft and translucent. Add the ground beef and cook until browned, breaking it up into small pieces. Stir in the mustard, parsley, chives, salt, pepper, marjoram, and nutmeg. Let the mixture cool. If it feels loose, stir in 1 to 2 tablespoons of breadcrumbs.
Make the dumpling dough.In a large bowl, combine the mashed cooked potatoes, squeezed raw potatoes, reserved potato starch, egg, flour, salt, pepper, and nutmeg. Mix gently until a soft dough forms. If the dough is too sticky to shape, add a little more flour or potato starch.
Shape the dumplings.With damp or lightly floured hands, take a portion of potato dough and flatten it into a thick round in your palm. Add a spoonful of beef filling to the center. Fold the potato dough around the filling and seal it completely, shaping it into a round or slightly oval dumpling.
Test one dumpling.Bring a large pot of salted water to a gentle simmer. Cook one small test dumpling first. If it holds together, continue with the remaining dumplings. If it breaks apart, add a little more flour or potato starch to the dough.
Cook the Gefillde Knepp.Gently lower the dumplings into simmering water. Do not let the water boil aggressively. Cook for 15 to 20 minutes, or until the dumplings float and feel firm but tender. Remove with a slotted spoon and drain well.
Serve hot.Serve the dumplings with sauerkraut, bacon cream sauce, browned butter and onions, mushroom gravy, or fresh herbs.