Select the 'Sauté' function and adjust to the medium setting. Add the butter and olive oil and stir until butter is melted and evenly distributed across the bottom.
Add onion slices and sprinkle with the garlic powder and Italian seasoning. Sauté, stirring occasionally, until the onions are soft and translucent, approximately 5 minutes.
Generously season the chicken thighs with salt and pepper and place in the Instant Pot smooth side down. Cook for 3-5 minutes until browned, then flip them over and repeat.
Add the stock, onion soup and white wine (if using), then use a spatula to carefully release any browned bits from the bottom of the pot.
Simmer until the liquid has reduced by around one half, then press 'Cancel' to end the Saute program.
Add the lid and lock into place, making sure the pressure release valve is set to 'Sealing' Select the 'Manual' button on the highest setting and adjust the cook time to 8 minutes.
When the cook time is complete, allow the pressure to naturally release for 5 minutes, then use quick release to open the Instant Pot.
Carefully place the cheese slices on top of the chicken thighs and place the lid back on the Instant Pot for 3-4 minutes, or until the cheese is thoroughly melted.
Notes
You can also use mozzarella, gruyère, Swiss, or comté cheeses. Sliced is best, but grated also works.