Begin by chopping the onion, carrots, and celery into small, even pieces to ensure they cook uniformly.
Heat a large heavy-bottomed pot over medium heat and add the olive oil, listening for a gentle sizzle.
Add the diced onion to the pot and cook, stirring occasionally, until it becomes translucent and fragrant, about 5 minutes.
Stir in the diced carrots and celery, cooking for another 5 minutes until they soften slightly and their aroma rises.
Push the vegetables to one side of the pot, place the chicken breasts in the cleared space, and pour in the broth to cover everything.
Bring the mixture to a gentle simmer, then reduce the heat to low and cook, uncovered, for about 20 minutes until the chicken is cooked through and the vegetables are tender.
Remove the chicken breasts from the pot and shred them using two forks, then return the shredded meat to the soup.
Add lemon zest and juice to the soup, stirring well to brighten the flavors, and season with salt, pepper, and thyme to taste.
Let the soup rest off the heat for a few minutes, allowing the flavors to meld and the broth to clarify slightly.
Ladle the hot, fragrant soup into bowls and garnish generously with chopped parsley for a fresh herbal finish.
