Season the chicken breasts generously with garlic powder, paprika, salt, and black pepper, rubbing the spices in by hand to ensure even distribution.
Pour olive oil into the Instant Pot and select the sauté function. Once the oil is hot, sear the chicken for 3 minutes, working in batches if necessary.
Flip the chicken with tongs and brown the other side for approximately 3 minutes. Transfer the browned chicken to a plate, set it aside, and scrape any browned bits from the bottom of the pot.
Add butter to the pot and sauté the onions until golden. Stir in the garlic and cook for 30 to 60 seconds until fragrant.
Deglaze the pot with chicken stock or broth, using a wooden spoon to loosen any remaining bits stuck to the bottom.
Return the chicken to the Instant Pot. Secure the lid, ensure the vent is sealed, and set to manual pressure cook for 7 minutes.
Allow the pot to reach pressure and cook. Once the timer ends, let the pressure release naturally for 2 to 3 minutes before manually releasing the remaining steam.
Thicken the sauce if desired by mixing 1 tablespoon of cornstarch with 1 tablespoon of water. Select the sauté setting, whisk in the slurry, and stir until the sauce thickens.
Stir in the lemon juice and zest, then garnish with fresh parsley and lemon slices. Serve warm over white rice or mashed potatoes.
