Place the chicken broth, chicken thighs, sliced celery, and unrinsed jasmine rice in a large Dutch oven or pot, then season lightly with salt and bring the mixture toward a simmer over medium-high heat. Once small bubbles gather steadily at the surface, lower the heat so the broth bubbles gently rather than rolling hard.

Simmer for 20 to 30 minutes, until the chicken is cooked through and the rice has softened enough to lose some of its distinct shape. The broth should look lightly thickened, and the chicken should be opaque and easy to separate with tongs.

While the soup cooks, finely chop the parsley, lemon zest, and up to ½ cup celery leaves, then grate in the garlic, season with salt, and stir the mixture together in a small bowl. The herb mixture should smell sharp, green, and lemony, ready to brighten the warm broth.

Lift out the chicken with tongs, shred it in a medium bowl with two forks, and stir it back into the soup before taking the pot off the heat; add the butter (optional), adjust the salt, and stir in lemon juice gradually. Stop adding lemon when the soup smells bright and tastes balanced without losing its chicken-forward character, then spoon it into bowls and finish with the parsley mixture.