Heat the olive oil in a large soup pot over medium heat until shimmering and fragrant.
Add the diced onion and minced garlic to the pot, stirring frequently. Cook for about 5 minutes until the onion is soft and fragrant, and the garlic becomes aromatic.
Stir in the chopped carrots and celery, cooking for another 3-4 minutes until they begin to soften and brighten in color.
Pour in the chicken broth, bringing the mixture to a gentle boil. Reduce the heat to a simmer and let it bubble gently for 5 minutes.
Add the shredded cooked chicken and rinsed rice to the pot, stirring to combine. Season with salt, pepper, and squeeze in the lemon juice.
Let the soup simmer gently for about 10-12 minutes, stirring occasionally, until the rice is tender and has absorbed some of the broth, thickening the soup slightly.
Taste the soup and adjust the seasoning with additional salt, pepper, or lemon juice as desired for brightness and balance.
Serve the hot soup in bowls, garnished with chopped fresh parsley for a pop of color and fresh flavor.