Turn on a multi-functional pressure cooker (such as Instant Pot(R)) and select Saute function. Heat olive oil and cook onion until translucent, about 2 minutes. Add Brussels sprouts and cook for 1 minute more. Sprinkle with salt and pepper; pour vegetable broth over Brussels sprouts. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 3 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove lid.