Chicken Egg-Drop Soup

INGREDIENTS

• 2 tablespoons mirin (sweet rice wine) or dry sherry
• 1 1-inch piece ginger, thinly sliced
• 3 large eggs, well beaten
• 4 scallions, roughly chopped
• 1 teaspoon toasted sesame oil
• 1/2 pound skinless, boneless chicken breasts, cut into 1/4-inch cubes
• Kosher salt and freshly ground white pepper
• 4 tablespoons cornstarch
• 4 cups low-sodium chicken broth
• 1 small bunch watercress, tough stems removed