Gather all your ingredients: shredded cooked chicken, sliced carrots, sliced celery, diced onion, noodles, bruised lemongrass, chopped herbs, lemon, and broth.
Heat a large heavy-bottomed pot over medium heat, adding the oil. Once shimmering, add the diced onion and sauté until translucent and fragrant, about 5 minutes, with a gentle sizzle and aromatic smell filling the kitchen.
Add the sliced carrots and celery to the pot. Cook for about 4 minutes, stirring occasionally, until the vegetables start to soften and release sweet, earthy scents.
Pour in the chicken broth and add the bruised lemongrass stalk. Bring the mixture to a gentle simmer, letting the broth bubble softly and fill your kitchen with a bright, lemony aroma.
Reduce the heat to low. Add the noodles and cook, stirring occasionally, until just tender, about 6-8 minutes. Taste and adjust seasoning with salt and pepper as needed.
Remove the lemongrass stalk from the soup to prevent overpowering the delicate broth, then stir in the shredded chicken. Let everything heat through for a couple of minutes.
Ladle the hot soup into bowls, then garnish generously with chopped fresh herbs and squeeze fresh lemon over each bowl for a bright, citrusy finish. Serve immediately and enjoy the comforting, vibrant flavors.
