Hot-and-Sour Shrimp and Lemon Grass Soup

1 pound medium shrimp, peeled and deveined, with shells reserved
4 cups water
3 stalks fresh lemon grass, green tops trimmed off and reserved, white bulb pounded flat and cut into 1-inch lengths
1 15-ounce can straw mushrooms, drained
1 small tomato, cut into thin wedges
1 large scallion, thinly sliced
1 ½ cups mung bean sprouts
1 ½ teaspoons grated lime zest
3 tablespoons plus 1 teaspoon fresh lime juice
¼ teaspoon freshly ground pepper
¼ cup bottled fish sauce (nuoc mam)
¼ teaspoon dried chili flakes, plus more to taste
½ teaspoon chili oil
4 cups cooked white rice

Nutritional Information
- Nutritional analysis per serving (2 servings)
  828 calories; 4 grams fat; 0 grams saturated fat; 0 grams trans fat; 1 gram monounsaturated fat; 1 gram polyunsaturated fat; 136 grams carbohydrates; 8 grams dietary fiber; 11 grams sugars; 63 grams protein; 365 milligrams cholesterol; 3990 milligrams sodium

Six first-course servings or two to three main-course servings

In a saucepan, cover the shrimp shells with 4 cups water. Add the lemon grass tops and bring to a boil over high heat. Remove, strain the broth and return it to the saucepan.
Add the mushrooms, lemon grass pieces and tomato. Bring to a boil, reduce the heat to medium and simmer for 4 minutes. Add the shrimp and simmer until they are opaque, about 2 minutes. Pour into a tureen. Stir in the scallion, bean sprouts, lime zest and juice, pepper, fish sauce, chili flakes and chili oil. Serve with the rice.

20 minutes