Start by bruising the lemongrass with the back of a knife or a meat tenderizer to release its aromatic oils. Cut it into 2-inch pieces. Separate the cilantro leaves from the stems, then rough chop the leaves and set them aside.

In a bowl, combine the minced cilantro stems, ground beef, 2 tablespoons of fish sauce, and 1 teaspoon of black pepper. Mix until everything is well incorporated.

Heat the olive oil in a large saucepan over medium heat. Add the shallots and lemongrass, cooking until the shallots are translucent, about 3 minutes. Add the beef mixture to the pan, stirring to break it into small pieces as it cooks.

Add 4 cups of water (or 6 if your pot allows), 1 teaspoon of salt, and bring the mixture to a boil. Reduce the heat to a lively simmer and cook for 10 minutes, allowing the flavors to meld together. Stir in the remaining fish sauce and adjust seasoning as needed.

Just before serving, stir in the watercress and rough chopped cilantro leaves. They’ll wilt beautifully in the hot broth, adding fresh flavor and a vibrant pop of color.