Mix together the onion, sugar and vinegar and set aside.
Put the water, salt and pasta in the Instant Pot and give it a stir. Close the lid, make sure the valve is set to Sealing and push the Pressure Cook (or Manual) button. Adjust time using the + and – buttons to get to 5 minutes.
When it beeps that it’s done, flip the valve from Sealing to Venting for a quick release and when the pin drops, remove the lid. Drain the pasta.
Whisk together the mayo, mustard, celery seed, salt and pepper. Add the onion/vinegar mixture and pour over the pasta. Add the celery and cheese cubes and stir well. Refrigerate for several hours to develop the flavour.
Pressure Cook and Manual buttons are interchangeable.