Select the 'saute' button on the Instant Pot. Add olive oil and onion to the stainless steel insert, and cook for 5-7 minutes, until soft and translucent.
Add garlic to the pot and cook for 1 minute.
Add stock, and use a spatula to scrape the bottom of the pot so there's nothing stuck.
Add the celery, broccoli, potato, salt and pepper.
Place the lid on the Instant Pot, set the steam release handle to the 'sealing' position, and pressure cook on high pressure for 5 minutes.
Allow the pot to naturally depressurize for 10 minutes after the cooking cycle completes. After 10 minutes, carefully set the steam release handle to the 'venting' position, to allow steam to release. Open the lid.
Add milk to the Instant Pot, and use an immersion blender to blend until smooth.
Stir in the cheese until melted, and enjoy!