Put the potatoes in a large pot filled with cold water. Bring to a boil, reduce the heat to medium and cook until fork-tender, 35 to 40 minutes. Let the potatoes cool for 45 minutes; they will still be warm but will hold their shape when sliced into 1/4-inch-thick rounds.
Heat 1 tablespoon of the oil in a large saute pan over medium heat. Add the bacon and cook until crisp, stirring occasionally, about 10 minutes. With a slotted spoon, remove the bacon to a paper-towel-lined plate; set aside. Remove the drippings from the pan; return 3 tablespoonfuls and discard the rest.
Add the onions to the pan and sweat over medium heat until soft, about 6 minutes. Add the vinegar, mustard, sugar, 2 1/2 teaspoons salt and 1 teaspoon pepper and cook until fragrant, 1 minute more.
Add the cooled potatoes, half the cooked bacon, half the scallions, half the parsley and the remaining 2 tablespoons olive oil to the pan. Stir to combine and season with salt and pepper. Serve garnished with the remaining bacon, scallions and parsley.
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