Turn your Instant Pot to a high saute.
When it says HOT, add the butter.
Once the butter melts, add the onion.
Cook for 5 minutes, stirring occasionally.
Add the broth, and scrape up any bits on the bottom of the Instant Pot.
Hit cancel.
Add the balsamic vinegar and Worcestershire sauce.
Season the chicken with garlic powder, onion, salt, and pepper.
Arrange the chicken on top of the onions as close to a single layer as possible.
Set the Instant Pot to manual high pressure and the time to 8 minutes.
Add the lid and set it to sealing.
When the time is up, allow the pressure to release naturally for 5 minutes, and then do a quick release.
Preheat the broiler.
Use tongs to transfer the chicken to a baking sheet. Use a slotted spoon to remove the onions from the Instant Pot and arrange them on top of the chicken.
Top with the cheese.
Broil until the cheese melts, about 3 minutes.
While the cheese is melting, whisk together the cornstarch with 1 tablespoon of water and stir into the instant pot. Set the pot to saute and cook for a few minutes until the sauce thickens.
Serve the chicken with the sauce.