Chop carrots, celery, and onion into small, even pieces to ensure quick and uniform cooking.
In a large heavy-bottomed pot, heat a splash of oil over medium heat. Add the diced onion and cook until it turns translucent and begins to smell sweet, about 5 minutes.
Add the diced chicken to the pot and cook until lightly browned on all sides, about 7 minutes, stirring occasionally.
Pour in the chicken broth and bring it to a gentle boil. Meanwhile, bruise the lemongrass stalks and add them along with sliced ginger to the pot for aromatic infusion.
Add chopped carrots and celery, then reduce the heat to a simmer. Cover the pot partially and let it simmer gently for 30 minutes, allowing the flavors to meld and the broth to develop a bright, fragrant aroma.
Taste the broth and add fish sauce to deepen the umami flavor, adjusting the amount to your preference.
Remove the lemongrass stalks and ginger slices from the pot, as they have released most of their aroma and flavor.
Give the soup a gentle stir to distribute the flavors evenly, then ladle the bright, fragrant broth and tender chicken into bowls.
Optionally, garnish with fresh herbs like cilantro or sliced green onions for an extra burst of freshness.
Enjoy your warm, revitalizing bowl of lemongrass chicken soup, savoring the layers of bright, spicy, and comforting flavors.
