Sauté aromatics: Set Instant Pot on Sauté mode on high heat. Add butter & oil. When the butter begins to melt throw in the chopped onion and garlic and sauté for just 20 seconds.
Sauté Vegetables & seasonings: Throw in the diced carrots & celery, sauté for 1 min. Add the seasonings except the fresh rosemary twig ( mentioned above in the "seasonings" section in the ingredients list) and sauté for 2 mins. If adding dried rosemary add it now. Also, add salt according to your taste preference.
Deglaze pot: Cancel the Sauté mode. Add chicken stock and deglaze the pot very well by scrapping the sides & the bottom of the pot with the help of your spatula. There should not be any food particles stuck at the bottom of the pot. This step is VERY important and will prevent the burn error.
Orzo Pasta & chicken: Now, add the orzo pasta and give everything a good stir. Place the thawed or fresh boneless skinless chicken thighs on top of the pasta and DO NOT stir of mix. Throw in the fresh rosemary twig & the freshly grated lemon zest.
Pressure Cook: Secure the lid of the instant pot, PRESSURE COOK on high for just 3 mins. Once, the cooking cycle is over the instant pot will begin to beep. Immediately perform a quick pressure release by moving the valve from the sealing to the venting position. Open the lid when the floating pin on the lid drops. Cancel the pressure cook option.
Shred Chicken: Fish out the fresh rosemary twig & discard. Also, fish out the cooked chicken & shred them with the help of a pair of fork. You can even dice it into bite size pieces if you prefer it that way.
Thicken soup: Add lemon juice into the pot, mix well. Add back the shredded chicken & give everything a good stir. Now, add the optional cornstarch slurry (Read **NOTES below), stir well & simmer the soup on Sauté set at low heat for just 1 min or until the soup thickens a bit. Serve immediately for best texture and enjoy! You can garnish with fresh chopped flat leaf parsley.