Add the chicken, lemongrass, ginger, carrot, celery, shallot, bay leaf, black peppercorns, Thai chile and 1 teaspoon salt to a large Dutch oven or soup pot. Add 6 cups cold water and place over medium heat. Bring the soup to a simmer, skimming off any residue that may come to the top. Reduce the heat to low; cover and cook for 45 minutes or until the chicken is cooked through. Turn off the heat and allow the soup to cool for about 30 minutes.
Remove the chicken from the soup and shred the meat, discarding the bones and skin. Strain the stock, discarding the solids, and return the liquid to the pot. Add the shredded chicken back into the stock, along with the spinach, and bring the soup to a simmer over medium-high heat. Serve immediately.
Lemongrass adds a subtle citrusy flavor to this soup. Smash the stalks with the flat side of a knife to release the oils.