Rub chicken with homemade rub: Add all the ingredients mentioned above in the "Chicken Rub" section into a bowl and whisk together. Sprinkle the rub on both sides of the boneless skinless chicken thighs and rub well. Set them aside.
Sear chicken: Set the instant pot on SAUTE & keep it on HIGH heat. Add butter, when the butter begins to melt add the chicken and sear both sides until the edges are golden and a few brown spots develop. Set them aside. You need to sear the chicken in two batches.
Saute aromatics: In the same pot add the chopped shallots and garlic & saute for 1 minute. Add the diced mushrooms and saute for another minute.
Deglaze pot: Pour the homemade stock (warm water + better than bouillon chicken base) and deglaze the pot very well scraping the sides & the bottom of the pot with the spatula. This will loosen any stuck food particles & prevent the BURN error. This step is very important, do not rush! CANCEL the SAUTE function.
Pressure Cook: Add the seasonings mentioned above in the "Seasonings for Creamy Sauce" section & stir to mix well. Now, transfer back the seared chicken into the pot and arrange them in a single layer on top of the mushrooms. DO NOT stir or mix. Secure the lid of the instant pot and PRESSURE COOK on HIGH for just 2 mins. Once the cooking cycle is over the instant pot will begin to beep. Immediately do a quick pressure release by manually moving the vent from the sealing to the venting position. Open the lid carefully when the metal pin on top the lid drops.
Make Creamy Sauce: Fish out the cooked chicken & set aside. Cancel the pressure cook option. Add the slightly warm heavy cream and stir well. Add the finely grated parmesan cheese and stir briskly until the cheese fully dissolves to form a creamy sauce. You can do a taste test and add few pinches salt according to your preference to help pop the flavors.
Thicken Sauce: Hit back the SAUTE function & set it on LOW heat. Simmer the sauce gently stirring frequently and thicken it to your personal preference. Or you can make a quick cornstarch slurry (refer the Thickener section above) and add this to the sauce, stir, simmer to thicken it. This will give a thicker sauce. This is an optional step. Otherwise you may simmer the sauce to cook it down a bit to thicken it.
Serve: Turn off the instant pot. Transfer back the cooked chicken into the pot. cover it with the creamy sauce. Let stand for 5 mins and than serve. Enjoy!