Heat a large heavy-bottomed pot over medium heat. Add a tablespoon of oil and sauté diced onion and minced garlic until fragrant and translucent, about 5 minutes, filling your kitchen with a sweet, roasted aroma.
Add sliced carrots, celery, and bruised lemongrass to the pot. Cook for another 10 minutes, stirring occasionally, until the vegetables are tender and release a fragrant aroma.
Pour in the chicken broth and bring the mixture to a gentle simmer. Add the shredded cooked chicken and the rice noodles. Cook for about 8 minutes, or until the noodles are just tender and slightly chewy.
Taste the broth and season with salt and pepper as needed. Squeeze in fresh lemon or lime juice to brighten the flavors, and stir gently to combine.
Remove the lemongrass stalks and discard. Ladle the hot soup into bowls, garnishing each with fresh herbs and a sprinkle of chili flakes if desired for added heat and freshness.
Serve immediately while the broth is steaming and the flavors are vibrant. Enjoy a bowl of this comforting yet lively chicken noodle soup that’s perfect for any day.