Heat the large pot over medium heat, then add diced onions and sliced carrots. Sauté until the vegetables are soft and fragrant, about 8 minutes, until the onions are translucent and carrots are tender.
Add the chicken stock to the pot and bring it to a gentle simmer. Meanwhile, season the chicken thighs with salt and pepper, then cut them into bite-sized pieces.
In a separate pan, heat a little oil and cook the chicken pieces until they are just cooked through and slightly golden, about 10 minutes. Remove from heat.
Add the minced garlic and lemon zest to the sautéed vegetables, cooking for another minute until fragrant.
Pour the cooked chicken into the simmering broth along with the lemon juice. Stir well and cook for another 5 minutes to meld the flavors.
Stir in the chopped fresh herbs and adjust seasoning with additional salt, pepper, and chili flakes if desired. Taste and adjust the brightness with more lemon juice if needed.
Once everything is heated through and flavors are balanced, ladle the bright, steaming soup into bowls, and enjoy immediately.
