Lemongrass Chicken Soup

Name: Lemongrass Chicken Soup
Ingredients: 2 pounds skinless chicken legs; 4 lemongrass stalks, white and pale-yellow parts only, smashed and chopped; 4 scallions, halved crosswise; 1 onion, halved lengthwise; 2 ounces ginger (1-inch piece), ½ thinly sliced and ½ julienned; 1 fresh Thai or serrano chile, seeded; 1 teaspoon black peppercorns; 10 cups water; 5 stems fresh cilantro, plus ⅓ cup leaves; 3 stems fresh mint, plus ¼ cup thinly sliced leaves; 1 ½ tablespoons fish sauce; 1 cup mung bean sprouts