Stir together the vinegar, salt and sugar until dissolved. Finely chop or grate the onion and add it to this mixture and set aside.
Peel the potatoes (or not, if you prefer) and cut into 1/2 inch cubes. Put 1 ½ cups water and the salt into the Instant pot and toss in the potatoes. Spray an oven-safe dish with cooking spray and crack the eggs into it. Put this dish on top of the potatoes. (Or just put the eggs in their shells on top of the potatoes, if you prefer.)
Close the lid and make sure the valve is set to Sealing. Push the Pressure Cook (or Manual) button. Adjust time using the + and – buttons to get to 4 minutes.
Whisk together one cup of the Miracle Whip or mayonnaise, dry mustard (or Dijon), curry powder, celery seed, salt and pepper. You can add the extra 1/4 cup of mayo after the salad is assembled if needed but start with 1 cup. The vegetables will release moisture as the salad chills and you don’t want it sloppy. Drain the onions and add them to the dressing along with all the diced vegetables and frozen peas (still frozen). Stir well.
When the Instant Pot beeps that it’s done, leave it for a 4 minute natural release and then flip the valve from Sealing to Venting for a quick release. When the pin drops, remove the lid.
Remove the eggs and score into dice with a paring knife. Drain the potatoes and run under cold water for a couple of minutes.
Scoop the diced egg into the vegetable mixture and add the drained potatoes. Stir everything together and then taste and add salt and pepper if needed. Cover and refrigerate for at least 2 hours.
Pressure Cook and Manual buttons are interchangeable.