Heat a large heavy-bottomed pot over medium heat and add the oil. Sauté chopped onions and minced garlic until fragrant and translucent, about 2-3 minutes. The aroma should be inviting, and the onions slightly golden.
Add sliced lemongrass stalks to the pot, bruising them gently with the back of a spoon to release their citrusy aroma. Cook for 1-2 minutes until fragrant.
Pour in the chicken broth and bring the mixture to a gentle simmer. Once bubbling, add the shredded chicken, sliced carrots, and sliced celery. Let everything simmer for about 10 minutes, or until the vegetables are tender and the broth is infused with aromatic flavors.
While the soup is simmering, cook the egg noodles separately in boiling water until just al dente, according to package instructions. Drain and set aside.
Remove the lemongrass stalks from the soup if you used whole stalks, or leave them if chopped. Stir in the cooked noodles and season the soup with salt and pepper to taste. Let simmer for another 2 minutes, allowing the flavors to meld.
Finish by squeezing fresh lemon juice into the soup, stirring gently to brighten the flavors and add a citrusy kick. Taste and adjust seasoning as needed.
Ladle the hot soup into bowls, garnish with fresh herbs if desired, and serve immediately for a warm, comforting, and fragrant meal.
