Set the Instant Pot to saute, and add in the butter until it is melted.
Once heated, add the onions to the melted butter, and toss to coat them well.
Cook the onions down on the saute function for about 5 minutes until they begin to soften.
Mix in the fresh thyme, balsamic vinegar and beef broth.
Place the lid on the Instant Pot, and be sure the release valve is set to the sealing position.
Set the Instant Pot to cook on manual setting for 20 minutes. Once it is finished, allow a slow release for 10 minutes before moving the release valve to the venting position to allow the additional pressure to release.
Set the Instant Pot back to the saute function, and saute for roughly 10 minutes until some of the liquid is cooked down, and the soup begins to thicken.
Meanwhile, preheat the oven to the broil setting.
Place 8 slices of baguette bread on a baking sheet, and top with some of the shredded cheese.
Broil the cheese bread until browned.
Top each bowl of french onion soup with a piece of cheese bread, and some of the remaining shredded cheese.