Sprinkle the skinless side of each chicken thigh with about 1 teaspoon salt (about ¼ teaspoon per thigh) and some black pepper. Use your hands to rub it all around the chicken meat and skin.
In a 6-quart Instant Pot, press the sauté button and set to “normal” or “medium”. Add the olive oil. Once the oil is hot, add the chicken, skin side down, and let cook undisturbed for about 5 minutes or until golden brown. Use a spatula to help release it from the pot and transfer to a plate.
Add onion, mushrooms, thyme, oregano, and remaining 1 teaspoon salt and cook for 3-5 minutes, or until softened, stirring occasionally. Make sure to scrape up any bits stuck to the bottom of the pot. Stir in the spinach (if using), and cook until wilted, about 1 minute.
Stir in the rice and water. Place the chicken thighs on top (skin-side-up). Lock the lid in place and set the vent to “sealing”. Select “manual pressure” (set to high) and set the timer for 10 minutes, then let it naturally release for 10 minutes before manually releasing. It’s best to let it sit for 5-10 minutes before serving so all of the juices can settle in.
Serve topped with a generous squeeze of lemon and plenty of fresh herbs. Sprinkle some parmesan on top of the rice, if you’d like.