Press Sauté button on your Instant Pot. When hot, add the butter, onion, mushrooms, garlic, salt, pepper, parsley, and sauté until onions are tender, about 2 minutes.
Add the chicken breast and sear on both side (2-3 minutes per side).
Add the chicken broth, lemon zest, and lemon juice.
Close the Instant Pot, cancel Sauté, and press Manual or Pressure Cook button, high, for 8 minutes.
When cook time ends, do a Natural Release. This process takes about 10-11 minutes.
Open the Instant Pot, remove the chicken breasts to a plate and set aside.
In a small bowl, add the water and cornstarch and mix well.
Press Sauté on your Instant Pot and slowly add the cornstarch slurry. Continue to stir with a wire whisk and bring to a boil. The sauce will thicken.
Return the chicken to the Instant Pot and coat with sauce.
Serve over egg noodles or rice or alongside roasted potatoes and green beans or roasted/grilled vegetables.