Preheat your oven to 375°F (190°C). Pat the chicken thighs dry with paper towels to remove excess moisture. Season them evenly with most of the salt and black pepper to build flavor before cooking.
Heat olive oil in a large Dutch oven over medium-high heat. Place the chicken skin-side down and let it cook undisturbed until the skin becomes deeply golden and crisp. Flip the chicken briefly, then transfer it to a plate while keeping the flavorful drippings in the pot.
Add butter to the chicken drippings and allow it to melt completely. Stir in the chopped onion and cook until softened and translucent. Add the minced garlic and cook just until fragrant, being careful not to let it brown.
Pour the uncooked long-grain rice into the pot with the aromatics. Stir continuously so every grain becomes coated with the flavorful fat. Toasting the rice for about a minute helps enhance its nutty flavor and improves texture.
Pour in the chicken stock and add the remaining salt and pepper. Stir gently while scraping up any browned bits stuck to the bottom of the pot. These caramelized pieces add tremendous flavor to the rice.
Arrange the partially cooked chicken thighs on top of the rice mixture with the skin facing upward. Keeping the skin exposed allows it to stay crisp while the rice cooks underneath. The chicken juices will naturally flavor the rice during baking.
Transfer the uncovered pot to the preheated oven. Bake until the rice has absorbed the liquid and the chicken is fully cooked through. If the rice absorbs liquid too quickly, add a little extra stock as needed during cooking.
Remove the pot from the oven and let it rest briefly. Sprinkle chopped parsley over the top and squeeze fresh lemon juice across the chicken and rice. The fresh citrus brightens all the savory flavors beautifully.
Spoon the rice onto serving plates and top with a crispy chicken thigh. Garnish with additional lemon wedges and parsley if desired. Serve immediately while the chicken skin is still wonderfully crisp and crunchy.