Heat the olive oil in the Instant Pot using the sauté function. Add the onion and mushrooms and sauté, stirring often, for 5 minutes or until softened.
Stir in the rice and garlic and sauté, stirring, for 2-3 minutes.
Pour in the wine and stir, scraping up any bits that are stuck to the bottom of the pot. Cook, stirring, until wine is absorbed, 2-3 minutes. Press Cancel to turn off the Instant Pot.
Stir in the broth and scrape up any bits that are stuck to the bottom of the pot. Press any rice that is stuck to the sides of the pot down into the broth. Then place the thyme sprigs on top of the rice and broth.
Close the Instant Pot lid and move the valve to the sealing position. Pressure cook on high pressure for 5 minutes. (The Instant Pot will take a few minutes to pressurize and then the cook time will start counting down.) As soon as the cook time ends, quick release the pressure by carefully moving the valve to the venting position.
Once all of the steam has escaped and the pin drops down, carefully open the Instant Pot lid. Remove and discard the thyme stems (most of the leaves will have fallen off).
Stir in the butter and grated Parmesan cheese. Season with salt and pepper to taste. Serve, with additional Parmesan cheese if desired.
