Preheat the oven to 350°F. In the Instant Pot, add the chicken, tomatillos, Anaheim pepper, jalapeno pepper, onion, cilantro, garlic, Mexican oregano, salt, and juice from 1 lime. Pour the chicken broth or water over the contents and stir until the seasonings are mixed in.
Set the Instant Pot to pressure cook and cook on high pressure for 20 minutes. When done release the steam completely (it will smell amazing!)
Remove the chicken from the pot and set aside. Using an immersion blender, blend the tomatillo mixture that remains until smooth. If you don't have an immersion blender carefully pour the contents into a regular blender and blend until smooth. Once blended place back into the Instant Pot.
Turn the Instant Pot to the saute function on medium heat. Add the corn starch and mix it in until incorporated. This will thicken the sauce slightly. Cook for 10 minutes, until sauce has slightly thickened. Set aside.
Using 2 forks, shred the chicken. Add the chicken to a medium sized bowl. Set aside.