Press Sauté – High/More for 20 Minutes. Add 3 tbsp of the butter. Stir until melted.
In a small bowl, combine the kosher salt and cornstarch. Coat the chicken breasts in the cornstarch mixture and set aside.
Sear chicken breasts until golden brown all over, about 5-8 minutes on each side. Set aside.
Add the ½ cup water to the bottom of the pot, scraping up any browned bits. Add the sliced onions and toss to evenly coat in water. Press Cancel and secure the lid on top.
Pressure Cook on High for 3 Minutes. Allow the pot to naturally release for 3 minutes before flipping the valve and releasing any remaining pressure.
Pour the cooked onions and their juices over a fine mesh strainer set on top a large bowl, collecting all the juices.
Press Sauté – High/More for 20 Minutes. Add 2 tbsp of butter to the inner pot and stir until melted. Add the strained onions to the inner pot and saute until the onions caramelize, about 10 – 15 minutes stirring frequently. Make sure to constantly scrape up any browned bits from the bottom of the pan to ensure the onions do not burn. Remove the final caramelized onions from the inner pot. Press Cancel on the Instant Pot.
Place the seared chicken breasts back in the inner pot. Evenly spoon the caramelized onions on top of each chicken breast.
Add 1 cup of reserved onion liquid to the inner pot with the beef stock, pouring around the sides of the inner pot. Sprinkle the minced thyme over the top.
Pressure Cook on High for 5 Minutes. Allow the pot to naturally release for 5 minutes, then flip the valve and release any remaining pressure.
Add the shredded gruyere to the top of the chicken, replace the lid, allow the residual heat to melt the cheese, about 5 minutes. Top with additional minced thyme as desired. Serve immediately. Enjoy!