Frozen veggies are a snap using the Instant Pot. Set it, and forget it!
Place a steamer basket in the pot.
Add 1 C. Water.
Pour frozen bag of Brussels Sprouts into the steamer basket.
Press "Manual" and set the pot for "0" or "1" (more tender) minutes.
Secure the lid and seal the steam valve.
When the pot is done you may do a quick release of pressure by opening the steam valve. However, if you forget it, the pot will switch to warm and keep your veggies ready until you are! I often don't release the steam until a few minutes have passed.