Slice the chicken breasts thinly, chop the carrots, and mince the garlic and ginger. Bruise the lemongrass stalks and slice them into small pieces.
Heat the vegetable oil in a large heavy-bottomed pot over medium heat until shimmering and fragrant. Add the sliced chicken and cook, stirring occasionally, until just browned, about 3-4 minutes.
Remove the chicken from the pot and set aside. In the same pot, add the minced garlic, ginger, and bruised lemongrass. Sauté for about 2 minutes until fragrant, the aroma should fill the kitchen and the mixture becomes slightly translucent.
Pour in the chicken broth and bring to a gentle simmer. Add the sliced carrots and cook for about 10 minutes until they become tender but still hold their shape.
Return the cooked chicken to the pot, stirring to combine. Add the cooked rice noodles and cook for another 2-3 minutes until the noodles are heated through and tender.
Stir in the soy sauce and season with salt and pepper to taste. Let the soup simmer for a minute to meld the flavors, then remove from heat.
Taste the broth and adjust seasoning if needed. Ladle the soup into bowls, ensuring each serving has a good mix of chicken, vegetables, and noodles.
Serve hot, garnished with fresh herbs if desired, and enjoy the bright, lively flavors of this quick, nourishing soup.
