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1
Place potatoes, 1 tablespoon salt, and water to cover in large saucepan or Dutch oven, bring to boil over high heat, then reduce heat to medium and simmer until potatoes are tender (thin-bladed paring knife can be slipped into and out of potatoes with little resistance), about 10 minutes.
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2
Reserve 1/2 cup potato cooking water, then drain potatoes; return potatoes to pot and cover to keep warm.
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3
While potatoes are cooking, fry bacon in large skillet over medium heat, stirring occasionally, until brown and crisp, about 5 minutes.
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4
With slotted spoon, transfer bacon to paper towel-lined plate; pour off all but 1/4 cup bacon grease.
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5
Add onion to skillet and cook, stirring occasionally over medium heat until softened and beginning to brown, about 4 minutes.
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6
Stir in sugar until dissolved, about 30 seconds.
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7
Add vinegar and reserved potato cooking water; bring to simmer and cook until mixture is reduced to about 1 cup, about 3 minutes.
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8
Off heat, whisk in mustard and pepper.
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9
Add potatoes, parsley, and bacon to skillet and toss to combine; adjust seasoning with salt.
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10
Transfer to serving bowl and serve.
