Set your instant pot to SAUTE. Add in the 2 tablespoons butter and chopped onion. Saute for 5 minutes until soft, stirring occasionally. Add in the garlic, and stir another 30 seconds until fragrant.
To the instant pot, add in the 4 cups chopped broccoli, 1 cup shredded carrots and 3 cups chicken stock.
Close with the lid, and make sure the vent is at SEALING. Hit CANCEL, then cook on HIGH PRESSURE for 1 minute. Will take about 5 minutes to reach pressure, then it will start counting down from 1. After the 1 minute is up, do a NATURAL RELEASE for 5 minutes followed by a QUICK RELEASE, where you move the valve to VENTING to release all the steam. Once all the steam is gone and the float valve has dropped, you can safely open the instant pot.
Squeeze all the water out of the thawed cauliflower rice using a nut milk bag, cheese cloths or clean dish towels.
To your high speed blender, add the cauliflower rice, 1 cup chicken stock and 1 cup heavy cream. Blend on high speed until it is a puree.
Add the cauliflower puree to your instant pot and stir to combine.
Add 8 ounces grated cheddar cheese, and stir until melted.
Season with kosher salt and pepper to taste.
Leftovers will keep in an airtight container refrigerated for at least 5 days, or frozen for at least 6 months.