Melt the butter in the Instant Pot using the SAUTE setting. Add onion and celery, and saute until softened (about 3 minutes). Add garlic and saute for 30 seconds. Switch off the saute setting.
Add diced potatoes, raw chicken breasts, and pour in the chicken stock.
Secure the lid and set the vent to SEALING. Select the PRESSURE COOK/MANUAL setting and set the cooking time for 7 minutes at high pressure. The Instant Pot will take around 7-8 minutes to come to pressure and start cooking. When the cooking program ends, quickly release the steam.
Carefully remove the lid, take the chicken out (put the lid back on to keep the soup warm) and shred or dice the chicken into pieces then return to the Instant Pot.
Stir in the heavy cream, Italian seasoning, garlic powder, and season with salt and pepper. Thicken the soup with cornstarch slurry, and finally stir in the frozen mixed vegetables. Cover with the lid for 5 minutes, this will help the frozen vegetables soften.
Give the soup a stir, ladle into bowls and garnish with freshly chopped parsley. Serve with homemade biscuits.