Preheat oven to 425 degrees and line two baking sheets with parchment. Slice potatoes 1/4 inch thin. Toss with olive oil, apple cider vinegar, dried dill, salt and pepper, then spread onto baking sheets with no overlap. Roast 25 minutes on one side, then flip and roast 20 more minutes until crispy.
While the potatoes roast, caramelize the onions. Melt butter in a skillet over medium-low heat, then add onion and a pinch of salt and pepper. Saute, stirring occasionally, for at least 30 minutes until onions brown and become jammy. You may need to lower the heat if the onions begin to crisp up too quickly. "Low and slow" is the name of the game!
To assemble the nachos, spread half of the onions, half of the sauerkraut, and half of the Swiss cheese over one layer of roasted potatoes. Add a second layer of potatoes, the rest of the onions, sauerkraut, and cheese. Bake for 8-10 more minutes until cheese melts and bubbles. Before serving, sprinkle with parsley.