Prepare the potatoes.Peel the potatoes, if desired, and cut them into small cubes, about 1/2 inch wide. If using raw potatoes, rinse briefly and pat very dry. If using cooked potatoes, let them cool completely before dicing.
Heat the fat.Place a large heavy skillet over medium-high heat. Add the lard, clarified butter, bacon fat, or oil. The fat should be hot enough that a potato cube sizzles when it touches the pan.
Fry the potatoes.Add the diced potatoes in an even layer. Do not overcrowd the pan. Let them cook without stirring for several minutes so the bottoms can brown.
Turn and continue cooking.Turn the potatoes with a spatula and continue sautéing until the cubes are golden on several sides. If using raw potatoes, reduce the heat to medium after the first browning, cover loosely, and cook until tender, stirring occasionally. This usually takes 18 to 25 minutes. Cooked potatoes will take closer to 10 to 15 minutes.
Add onion or bacon, if using.If adding bacon, cook it first in the skillet until the fat renders, then fry the potatoes in the bacon fat. If adding onion, stir it in once the potatoes are mostly tender so it softens and browns without burning.
Season the potatoes.Season with salt, pepper, marjoram, and a small pinch of nutmeg, if using. Toss gently so the seasoning coats the potatoes.
Finish with herbs.Sprinkle with parsley and chives just before serving. Taste and adjust salt as needed.
Serve hot.Serve the Kerschdscher straight from the pan while the edges are crisp. Pair with salad, applesauce, sausage, eggs, sauerkraut, or pickles.