Season the chicken thighs thoroughly with garlic powder, paprika, oregano, salt, and black pepper. Set the Instant Pot to the SAUTE function, add the olive oil, and melt the butter completely. Sear the chicken thighs starting with the skin side down for 2–3 minutes until golden, then turn and brown the other side for an additional 2 minutes. Remove the browned chicken and set aside, working in batches if required. Pour in the chicken stock and deglaze the pot, ensuring any browned bits are lifted from the bottom. Position a trivet inside the pot and arrange the chicken thighs atop it, allowing slight overlap if needed. Fasten the lid securely, set the valve to SEALING, and select PRESSURE COOK or MANUAL for 10 minutes on high pressure. The Instant Pot will take about 6–7 minutes to pressurize. Once the cooking cycle ends, let the steam release naturally for extra tender meat, or use quick release if preferred. Transfer the chicken thighs to a serving dish. To craft a gravy, activate the SAUTE mode, combine 1 tablespoon of cornstarch with 1 tablespoon of water, stir into the cooking liquid, and whisk until the sauce thickens. Adjust seasoning if necessary, then drizzle the sauce over the chicken and garnish with freshly chopped parsley.