Cook the brown rice according to package directions and set it aside.
In a large pot, combine the vegetable broth, bruised lemongrass stalks, minced ginger, minced garlic, and sliced shallot. Bring the mixture to a simmer over medium heat.
Add the sliced mushrooms and cook for 5 minutes until they soften.
Pour in the light coconut milk and let the soup simmer gently for 5 more minutes without boiling.
Remove and discard the lemongrass stalks. Stir in the lime juice, sliced Thai chili, and salt to taste.
Divide the cooked brown rice among four bowls and ladle the soup over the top.
Sprinkle each bowl with chopped cilantro just before serving.