Turn Instant Pot to saute and add olive oil, diced chicken and onions. Cook until outsides of chicken are cooked/whitish in color. (if on the stovetop do the same in a pot on low/med heat)
Turn pot OFF/Cancel and scrape bits of chicken off the bottom of the pot to avoid burn message being triggered (can add a bit of broth to make sure all bits are off/loosened).
Add celery and chicken broth and close lid and steam valve.
Set to pressure high for 20 minutes. (if on the stovetop set on low heat for about the same time, cook until vegetables soften and chicken is done in the middle, then follow next steps with heavy cream)
Do a quick release and add heavy cream. Stir and if you'd like it a bit thicker add 3 tbsp of cold water into a small bowl with 1-2 tbsp cornstarch, whisk together until smooth and add back into pot on saute function until bubbling to thicken a bit. Serve.