Start by chopping the onion and slicing the carrots into thin rounds to prepare the vegetables.
Bruise the lemongrass stalks with the back of your knife to release their aroma, then chop into 3-inch pieces.
Heat a large heavy-bottomed pot over medium heat and add a splash of oil. Sauté the chopped onion until translucent, about 5 minutes, until it begins to smell sweet and turns slightly golden.
Add the minced garlic, grated ginger, and bruised lemongrass to the pot. Cook for about 2 minutes until fragrant, with a lemony and spicy aroma filling the air.
Pour in the chicken broth and bring the mixture to a gentle simmer. Skim off any foam that rises to keep the broth clear and bright.
Add the chicken thighs to the broth. Reduce the heat to low and gently simmer for 15-20 minutes until the chicken is just cooked through and tender.
Remove the chicken from the pot, shred it into bite-sized pieces, and return it to the broth. Add the sliced carrots and chopped bok choy, then let everything simmer for another 10 minutes until the vegetables are tender.
Taste the broth and adjust seasoning with salt, pepper, or a squeeze of lime for brightness. Add the sliced green onions and chopped herbs just before serving for a fresh finish.
Serve the hot soup in bowls with lime wedges on the side, allowing everyone to add a zesty squeeze just before eating. Garnish with additional cilantro and Thai basil for full flavor.