Heat the Instant Pot to high on the sauté setting. Add the butter, onion, celery, carrots, and mushrooms and sauté, stirring frequently, for 4-5 minutes, or until the onion is translucent.
Add the garlic, dried dill, dried parsley, onion powder, garlic powder, and frozen green peas to the pot. Saute for 2-3 minutes, or until nice and fragrant. Turn off the saute setting.
Lock on the pressure cooking lid and pressure cook on high for ZERO (yes, 0!) minutes. Natural-release the pressure for 5 minutes and then quick-release the rest of the pressure.
In a small mixing bowl, whisk together the all-purpose flour, and whole milk to make a slurry. Stir the slurry and the shredded cheddar cheese into the tuna mixture.
Turn the sauté feature onto high heat and bring the mixture to a boil, stirring regularly, or until the sauce has just thickened, or for about 1-3 minutes.