Start the instant pot in SAUTE mode and melt butter in it. Add diced onions and minced garlic and saute for 2 minutes.
Add broccoli, carrots, seasoning and broth. Mix it all well. Press Cancel and close the instant pot lid with vent in sealing position.
Set the instant pot on MANUAL or pressure cook mode at high pressure for 5 minutes. When the instant pot beeps, let the pressure release naturally. Open the lid.
When the pressure cooking is done, melt butter on a stovetop saucepan. Add the flour and saute for a minute until it is cooked. Whisk in the milk, a little bit at a time, until you have a smooth thick consistency.
Stir in the roux prepared on stovetop and the cheddar cheese gradually, stirring continuously, until it melts.
Change the instant pot to SAUTE mode and let the soup simmer for a couple of minutes.
Soup is ready to be served. Garnish with some grated cheese, black pepper or red chili flakes and enjoy with crusty bread.