Turn the Instant Pot on Sauté. Once the pot is hot, add butter, garlic, red pepper flakes, thyme, and sauté for 30 seconds.
Add onions, mushrooms, salt, pepper, and water and stir.
Put in the pasta and push down into the water as much as possible.
Place the mascarpone cheese on top of the pasta. Do not stir otherwise your cheese could stick and burn.
Secure the lid on the pot. Close the pressure-release valve. Select PRESSURE COOK and set the pot at HIGH pressure for 6 minutes. Allow the pot to sit undisturbed for 5 minutes, then release any remaining pressure.
Open the lid and stir. Stir in the parmesan cheese and mix well, allowing the mixture to thicken.
Garnish with thyme and serve.
When I say don't stir after adding the mascarpone cheese to the Instant Pot, I really mean don't stir! That is unless you like the flavor of burnt cheese in your dishes.
Serve with a simple salad and some crusty bread on the side to sop up all that delicious sauce.
If you are having a hard time finding mascarpone cheese (or if you just don't feel like going to the grocery store), you can use cream cheese mixed with heavy whipping cream in its place. Simply mix 8 ounces of room temperature cream cheese with a 1/4 cup of heavy whipping cream and mix well. I do highly recommend picking up some mascarpone cheese though, as it gives the Mushroom Pasta a richer taste.