Pour the vegetable broth into a large pot and add the bruised lemongrass, minced garlic, and sliced ginger. Bring the mixture to a gentle simmer over medium heat.
Let the broth simmer for 15 minutes so the lemongrass and ginger release their flavor into the liquid.
Remove the lemongrass pieces and ginger slices with a slotted spoon or small strainer.
Stir in the fish sauce and lime juice, then taste and adjust the seasoning if needed.
Add the chopped watercress to the pot and cook for 1 to 2 minutes until the leaves just wilt.
Ladle the soup into bowls and top each serving with sliced green onions, cilantro leaves, and a few pieces of Thai chili if using.