Melt 1 tablespoon of the butter in Instant Pot on SAUTÉ setting. Add chicken; cook and stir 3 minutes. Add onions and mushroom; cook and stir 2 minutes or until softened. Mix stock and Sauce Mix in medium bowl until well blended. Stir stock mixture and rice into pot. Close lid. Set Valve to Seal.
Select PRESSURE COOK (MANUAL); cook 7 minutes on HIGH PRESSURE. When done, quick-release pressure. Open lid once pressure inside the pot is completely released. (Check manufacturer’s manual for safe operating instructions.) Stir in cheese and remaining 1 tablespoon butter. Sprinkle with parsley and additional Parmesan cheese, if desired.