Pat the chicken thighs dry with paper towels. In a large mixing bowl, combine the olive oil, lemon zest and juice, minced garlic, chopped rosemary, thyme, paprika, salt, and pepper. Add the chicken thighs and toss until they are evenly coated. Let marinate at room temperature for at least 15 minutes, or cover and refrigerate for up to 4 hours.
Preheat your oven to 400°F (200°C). Heat a large, oven-safe skillet (at least 12 inches) over medium-high heat. Place the marinated chicken thighs skin-side down in the hot skillet. Sear for 4-5 minutes, without moving them, until the skin is deeply golden brown and crispy. Flip the chicken and sear for another 2 minutes on the other side. Transfer the seared chicken to a plate and set aside.
Reduce the heat under the skillet to medium. Add the butter and chopped onion to the chicken drippings in the pan. Sauté for 3-4 minutes, stirring occasionally, until the onion is soft and translucent. Scrape up any browned bits from the bottom of the pan.
Add the rinsed rice to the skillet and stir constantly for 1-2 minutes until the grains are lightly toasted and smell fragrant. This step helps the rice grains stay separate and fluffy.
Pour in the warm chicken broth and bring the mixture to a gentle simmer, stirring to combine. Turn off the heat.
Nestle the seared chicken thighs (skin-side up) back into the rice and broth mixture. Arrange the thin lemon slices on top of and around the chicken. Cover the skillet tightly with an oven-safe lid or a double layer of aluminum foil.
Transfer the skillet to the preheated oven and bake for 25-30 minutes, or until the rice is tender and has absorbed all the liquid, and the chicken is cooked through (internal temperature reaches 165°F or 74°C).
Carefully remove the skillet from the oven. Uncover and sprinkle the frozen peas (if using) over the rice. Immediately cover the skillet again and let it rest undisturbed for 10 minutes. The residual heat will cook the peas perfectly and allow the rice to finish steaming.
Before serving, fluff the rice with a fork. Garnish generously with fresh chopped parsley. Serve the chicken and rice directly from the skillet.