Pour the chicken stock into a large saucepan and bring it to a gentle simmer over medium heat, watching for a soft bubbling that signals it's ready for the next step.
Add the bite-sized chicken pieces into the simmering broth, stirring gently. Cook for about 8 minutes until the chicken is just opaque and tender, filling the kitchen with a savory aroma.
Meanwhile, heat the olive oil or melt the butter in a separate pan or directly in the pot, then toss in the diced carrots, celery, and onion. Sauté for about 5 minutes until the vegetables are fragrant and starting to soften, with a slight sizzle and a gentle aroma.
Transfer the sautéed vegetables into the simmering broth, stirring to combine. Continue simmering for another 10 minutes until the vegetables are tender but still hold their shape, and the broth is fragrant.
Stir in the dried thyme, chopped parsley if using, and season with salt and pepper. Let the soup simmer for another 5 minutes so the flavors meld together beautifully.
Taste the broth and adjust seasoning as needed. For a bright, fresh note, squeeze in a little lemon juice right at the end, stirring well to brighten the flavors.
Once everything is heated through and flavorful, ladle the steaming soup into bowls. Garnish with extra parsley if desired, and serve hot for a comforting, nourishing meal.
