Heat the olive oil in a large pot over medium heat, until it shimmers and you hear a gentle sizzle.
Add the diced onion and cook, stirring occasionally, until translucent and fragrant, about 3-4 minutes. The onions should be soft and slightly golden.
Stir in the sliced carrots and celery, cooking for another 5 minutes until they soften slightly and their aromas deepen.
Pour in the chicken broth, bringing the mixture to a gentle simmer. Once bubbling, add the shredded chicken and stir to combine.
Let the soup simmer gently for 15 minutes, allowing flavors to meld and vegetables to become tender.
Taste the broth and season with salt and pepper as needed. Squeeze in fresh lemon juice to brighten the flavors.
Stir in chopped parsley or dill for a fresh herbal note, and let it sit for a minute to release its aroma.
Serve the soup hot, with the shredded chicken, tender vegetables, and a bright, flavorful broth in each bowl.