In a big soup pot with the heat off, add the chicken and shrimp, plus all the ingredients under "Soup" except for the cornstarch, water, green chilies, and scallions.
For the lemongrass, discard the tough outer layer and use a knife to lightly bruise up the inner flesh to release flavor. Then cut into 2 inch pieces, peel and separate the layers, and add to the pot.
In a small bowl, dilute the cornstarch with 1/2 cup of water to form a smooth slurry. Add to the pot.
Mix everything in the pot extremely well.
Pour in 9.5 cups water, give another good mix, and turn on the heat. Bring it to a boil, then allow it to simmer for 10 minutes.
Add green chilies and adjust seasonings as necessary. Serve into bowls and garnish with scallions. Enjoy!