In the Instant Pot, combine the chicken, green beans, peas (if using), onion, celery, carrots, butter, garlic, pepper, thyme, salt, and broth.
Secure the lid on the pot. Close the pressure-release valve. Select PRESSURE COOK OR MANUAL and set the pot at HIGH pressure for 2 minutes. At the end of the cooking time, allow the pot to sit undisturbed for 5 minutes, then release any remaining pressure.
Open the lid, stir in the evaporated milk, and serve.