Step 1

Halve the potatoes (quarter if large), then cut crosswise into 1/2-inch-thick slices. Place in a large saucepan of heavily salted water and bring to a boil over medium-high heat. Reduce the heat to medium and simmer until the potatoes are knife-tender, 4 to 5 minutes. Meanwhile, cook the bacon and make the dressing.

Step 2

Place the bacon in a medium frying pan and cook over medium heat, stirring occasionally, until crisp and browned, 15 to 20 minutes. Using a slotted spoon, transfer the bacon to a paper towel-lined plate and set aside. Measure out 1/4 cup of the bacon fat and place in a large heatproof bowl.

Step 3

Add the vinegar, mustard, scallions, and 1/2 teaspoon salt to the bacon fat, season with pepper, and whisk to combine; set aside until the potatoes are ready.

Step 4

Drain the potatoes well, immediately add to the bowl of dressing, and gently toss with a rubber spatula to combine. Taste and season with more salt and pepper as needed (the potatoes will probably need a lot of salt). Set aside to cool slightly and for the flavors to meld, about 20 minutes. Add the parsley and reserved bacon and toss to combine. Serve warm or at room temperature.