Place the potatoes in a large pot of water. Set over high heat and bring to a boil. Once boiling, cook until fork-tender, about 6-10 minutes. Then drain off the water. Chop the soft potatoes into bite-size pieces. Meanwhile, place the bacon in a large saute pan. Set over medium heat and cook until the bacon is red-brown. Then pour off half the bacon grease and add the onions. Saute until the onions are soft and starting to caramelize around the edges. Add the apple cider vinegar, 1/2 cup of water, dijon mustard, and sugar. Whisk to combine. Then pour the chopped potatoes into the pan. Toss well to coat. Taste, then season with salt and pepper as needed. Toss in the fresh parsley and serve warm!