In a blender, combine the lime zest and juice, fish sauce, brown sugar, curry paste, garlic, yellow onion, chicken broth, and half the cilantro. Process until smooth. Salt the chicken thighs all over. Add the chicken thighs to a large, heavy saucepan or Dutch oven and pour over the purée, entirely submerging the chicken pieces. Bring to a boil over medium-high heat, then reduce the heat to low, cover, and simmer until the chicken is very tender and cooked throughout, about 30 minutes. Uncover and, using a slotted spoon, transfer the chicken to a cutting board to cool slightly. Meanwhile, add the green beans, bell pepper, green onions, and coconut milk to the pan and stir to combine. Bring to a simmer again over medium heat, cover, and cook until the vegetables are tender, about 10 minutes. While the vegetables are cooking, and as soon as the chicken is cool enough to handle, remove the bones, along with any fat or gristle, and discard them. Break the chicken into pieces and, when the vegetables are done, return the chicken to the pan. Add the remaining cilantro, cover again, and heat until the chicken is warmed through, about 10 minutes. Transfer to a warmed serving platter, garnish with cilantro leaves, and serve with steamed rice.