Press SAUTE on Instant Pot. Add butter (or oil), and once it’s hot, add onions and minced garlic. Saute until the onions soften.
Then add potato cubes, vegetable broth (or water), salt, and pepper. Mix well and deglaze the pot by scraping the bits stuck to the bottom to avoid a BURN notice.
Close the lid on the pot. Set the pot to MANUAL/PRESSURE COOK (High Pressure) and set the timer to 6 minutes.
While the soup cooks, in a small bowl, whisk together 2 tablespoons of water (or milk) with 2 tablespoons of cornstarch (or flour) to make a slurry. Keep aside.
When the instant pot beeps, Allow 5 minutes Natural pressure release and then do a quick pressure release.
Open the lid, Add the cornstarch slurry, milk, mix well and bring the soup to a boil and simmer for a few mins. Stir often.
Take a potato masher and lightly mash potatoes to get desired (creamy and yet chunky) consistency.
Lastly, add heavy cream, and cheddar cheese. Give everything a good mix until the cheese melts—season with salt and pepper to taste if needed.
Serve cheesy potato soup immediately with top it with shredded cheddar and green onions.