Gather all ingredients and tools before starting to ensure a smooth cooking process.
Heat a large heavy-bottomed pot over medium heat and add oil, letting it warm until shimmering and fragrant.
Add diced onion, minced garlic, and sliced ginger to the pot. Sauté until fragrant, about 3 minutes, until the mixture becomes translucent and aromatic.
Stir in sliced carrots and celery, cooking for another 3-4 minutes until they begin to soften and release their aroma.
Place the chicken breasts or thighs into the pot, then cover with cold water, ensuring the meat is submerged. Bring to a gentle simmer, skimming off any foam that rises to the surface.
Bruise the lemongrass stalks slightly and add them to the simmering broth, along with a pinch of salt and pepper. Cover partially and cook gently for 20-25 minutes until the chicken is cooked through and tender.
Remove the chicken from the broth and place on a cutting board. Shred the meat into bite-sized pieces using two forks or your hands.
Discard the lemongrass stalks and return the shredded chicken to the pot. Stir well to combine with the broth and vegetables.
Squeeze fresh lemon juice into the broth to brighten the flavors, then taste and adjust seasoning with salt and pepper as needed.
Let the soup simmer for another 10 minutes to allow flavors to meld and vegetables to finish softening.
Ladle the hot soup into bowls, garnish generously with chopped cilantro and Thai basil, and serve immediately for a fragrant, lively meal.
