Step 1

1 Place the potatoes in a large pot; add enough cool water to cover by several inches

Step 2

2 Bring to a boil over medium-high heat; cook for 15 minutes (small potatoes) to 30 or 45 minutes (larger ones)

Step 3

3 Test for doneness by inserting a thin skewer, which should meet with little resistance

Step 4

4 Meanwhile, heat 1 tablespoon of the oil in a small nonstick skillet over medium-high heat until the oil shimmers

Step 5

5 Add the diced onion and a light sprinkle of salt; stir to coat

Step 6

6 Reduce the heat to medium, further adjusting as needed to keep the onion from browning

Step 7

7 Cook for 6 to 8 minutes, until the onion is tender

Step 8

8 Whisk together the parsley, vinegar and mustard in a mixing bowl

Step 9

9 Season with salt and pepper to taste

Step 10

10 Slowly whisk in the remaining 4 tablespoons of oil

Step 11

11 Taste, and adjust the seasoning as needed

Step 12

12 When the onions are done, stir them into the parsley dressing

Step 13

13 Use a slotted spoon to transfer the potatoes to a cutting board

Step 14

14 Let them cool for 5 minutes, then cut them into 1/2-inch cubes

Step 15

15 Immediately add them to the onion-parsley dressing mixture, tossing to combine

Step 16

16 Taste, and adjust the seasoning as needed

Step 17

17 Serve warm or at room temperature