Heat 2 tablespoons olive oil in a 6-quart Dutch oven over medium heat; add the diced onion, celery, and carrots and cook until the onion is translucent, about 6–8 minutes.

Stir in the minced garlic and cook for 30 seconds until fragrant, then add 8 cups chicken broth and 1 bay leaf, scraping up any browned bits.

Add the bone-in chicken thighs to the pot, bring to a gentle simmer, reduce heat to low, cover, and simmer until the chicken reaches 165°F and pulls away easily, 25–30 minutes.

Remove the chicken to a plate and let cool slightly; taste the broth and add 1 to 1 1/2 teaspoons kosher salt if needed.

Add 1 cup rinsed long-grain white rice to the simmering broth, return to a gentle simmer, and cook uncovered for 18–20 minutes until the rice is tender but separate.

Shred the cooled chicken with two forks, discard bones, return the shredded meat to the pot, stir in 2 tablespoons chopped parsley and 2 tablespoons lemon juice if using, warm 2–3 minutes, adjust seasoning, and serve.

If using a slow cooker, sauté the diced onion, celery, and carrots in 2 tablespoons olive oil in a skillet over medium heat until softened, about 6 minutes; transfer to a 6-quart slow cooker.

Add 8 cups chicken broth, 1 bay leaf, 1 to 1 1/2 teaspoons kosher salt, 1/2 teaspoon black pepper, and the bone-in chicken thighs to the slow cooker and cook on low for 6–8 hours or high for 3–4 hours until the chicken is very tender.

Remove the chicken, shred and discard bones, strain or skim the broth if desired, then return shredded chicken to the slow cooker.

Par-cook 1 cup long-grain rice on the stove for 8–10 minutes (it should be slightly undercooked), drain, and add to the slow cooker 20–30 minutes before serving so the rice finishes cooking without becoming mushy.

Stir in parsley and lemon juice if using, taste and adjust salt, and serve.