Chop the onion and dice the carrots, setting them aside for the next step.
In a large heavy-bottomed pot, melt the butter over medium heat until it begins to sizzle softly.
Add the chopped onion and diced carrots to the pot, cooking until they soften and the onion becomes translucent, about 5 minutes. Stir occasionally and listen for a gentle sizzle.
Shred the cooked chicken thighs or breasts into bite-sized pieces, then add to the pot, stirring to combine with the vegetables.
Pour in the chicken broth and bring the mixture to a gentle simmer, around 80°C or until small bubbles form, stirring occasionally.
Add the rice to the simmering broth, stirring once to prevent sticking. Cover partially and cook for 20-25 minutes, until the rice is tender and the soup has thickened slightly.
While the rice finishes cooking, roll the lemon on the counter to loosen the juice, then cut in half and squeeze out the fresh lemon juice into a small bowl.
Remove the soup from heat, then stir in the lemon juice, chopped parsley, and thyme. Taste and season with salt and pepper as needed.
Serve the soup hot, garnished with additional herbs if desired. The soup should be fragrant, with tender rice and shredded chicken floating in a bright, flavorful broth.
