In a large pot, combine the chicken broth, crushed lemongrass, lime leaves, ginger slices, minced garlic, and Thai chilies. Bring to a simmer over medium heat.
Once simmering, add the sliced chicken thighs to the pot. Cook until the chicken is no longer pink, about 5-7 minutes.
Stir in the sliced mushrooms and julienned carrots. Allow them to cook in the broth for about 3-4 minutes.
Pour in the coconut milk and stir gently. If using, add the baby spinach or bok choy at this stage. Let the soup simmer for another 2-3 minutes until everything is warmed through.
Season the soup with fish sauce, lime juice, salt, and pepper to taste. Adjust the seasoning based on your preference.
Remove the soup from heat and discard the lemongrass stalks and lime leaves.
Ladle the soup into bowls, garnish with fresh cilantro leaves, and serve hot.
Notes
Adjust the number of chilies based on your spice preference.