Heat a large heavy-bottomed pot over medium heat and add a splash of oil. Sauté chopped onions until they become translucent and fragrant, about 5 minutes.
Add minced garlic, grated ginger, and bruised lemongrass to the pot. Cook until fragrant, about 2 minutes, filling your kitchen with a lemony, spicy aroma.
Pour in the chicken broth and bring to a gentle simmer. Skim off any foam that rises to keep the broth clear and bright.
Add chicken thighs to the broth, ensuring they are submerged. Cover and cook gently for 15-20 minutes until the chicken is cooked through and tender.
Remove the chicken from the pot, shred it with two forks, and set aside.
Add sliced carrots and chopped bok choy to the broth. Let the soup simmer until the vegetables are tender, about 10 minutes.
Return the shredded chicken to the pot, stirring to combine. Taste the broth and adjust seasoning with salt, pepper, and a squeeze of lime for brightness.
Just before serving, add sliced green onions, chopped cilantro, and Thai basil to the soup. Let the herbs wilt slightly, releasing their vibrant aroma.
Squeeze fresh lime juice into the soup for an extra punch of citrus flavor. Ladle into bowls and serve hot, with rice or crusty bread if desired.