Place the inner pot inside Instant Pot. Plug it in. Press Saute Mode and set it on high. Once the screen displays HOT, add butter to it. As the butter melts, add chopped onions, minced garlic and chopped celery to the pot. Stir and Saute for about 2 minutes, till onions become soft.
Now add thickly grated carrots, broccoli florets and vegetable broth to the pot. Give it a stir and add salt + pepper to taste. Stir again. Cancel Saute Mode. Close the lid and set it to SEALING. Press Pressure Cook Mode and set it on HIGH for 3 minutes.
Once the timer goes off, follow QPR (quick pressure release). Open the lid and smush the vegetables well with the back of the kitchen ladle. If you like it chunky, then muddle it lightly. Set aside.
Now add cold milk to the corn starch and mix well with a whisk or spoon. Make sure that no lumps are left behind. Add this slurry to the cooked vegetables. Press Saute Mode on high and simmer for about 3-4 minutes till it thickens.
Lastly, add 1 cup of shredded cheddar cheese to the pot. Stir and cook for 2 minutes till cheese is all dissolved.
Hot Broccoli Cheddar Soup is ready to be served.