Chop the onion, carrots, and celery into small, even pieces, and mince the garlic. Zest the lemon and set both the zest and juice aside.
Heat olive oil in a large pot over medium heat. Add the chopped onion and garlic, sautéing until fragrant and translucent, about 5 minutes, with a gentle sizzle and a scent of sweetness filling the air.
Add the diced chicken to the pot, stirring occasionally until it begins to brown on the outside, about 7 minutes. The meat should look opaque and start to develop a light golden color.
Pour in the chicken broth, then add the diced carrots and celery. Bring the mixture to a gentle simmer, and cook uncovered for 15 minutes, allowing the vegetables to soften and flavors to meld.
Stir in the lemon juice and lemon zest, adjusting seasoning with salt and pepper. Let the soup simmer for another 5 minutes, so the bright citrus flavors infuse the broth and the chicken is fully cooked through.
Garnish the soup with chopped fresh parsley and thyme, adding a burst of herbal freshness that enhances the brightness of the dish.
Serve the lemon chicken soup hot, with a slice of crusty bread if desired. The clear, vibrant broth with tender chicken and colorful vegetables makes for a comforting and lively meal.