Season the chicken thighs on both sides with sea salt, black pepper, and 1/2 teaspoon garlic powder.
Set the Instant Pot to Sauté (Normal). Add 2 tablespoons olive oil and heat until shimmering.
Sear the chicken in batches for about 2 minutes per side, until golden brown; remove browned chicken and set aside.
Add 1 to 2 tablespoons butter to the pot, then sauté the chopped onion and sliced or minced garlic until softened, about 1–2 minutes.
Pour in the lemon juice and use a spatula to deglaze the bottom of the pot, scraping up any browned bits.
Stir in the Italian seasoning, lemon zest, chicken broth, and the remaining tablespoon of butter.
Return the seared chicken to the Instant Pot. Lock the lid and set the valve to SEAL.
Select Manual or Pressure Cook and set the timer to 7 minutes; the pot will take about 5–10 minutes to come to pressure.
When the cook time finishes, allow a natural release for 2–3 minutes, then carefully quick-release any remaining pressure and open the lid.
If a thicker sauce is desired, make a slurry with 1 teaspoon cornstarch whisked into 2 teaspoons cold water, stir into the sauce, press Sauté, and simmer until slightly thickened.
Spoon sauce over the chicken and garnish with chopped parsley and lemon slices before serving.
