1. In a large pot bring the broth to a strong simmer over medium-high heat. Cut 2 of the garlic cloves into quarters and add to broth. Add lemongrass, ginger, serrano chili, chopped cilantro, and fish sauce. Return to strong simmer, cover, adjust heat to medium-low, and simmer to infuse flavors,about 15 minutes.
2. Add the chicken and simmer until meat is opaque and cooked through, about 15 minutes. Transfer chicken to a plate; when cool enough to handle, either chop or pull apart into thin shreds (there should be about 2 cups). While chicken cools, strain broth into a large heat proof bowl, pressing solids to release as much liquid as possible; discard solids.
3. Wipe pot clean, add oil, and heat over medium heat. Add shallots, bell pepper, and salt and cook, stirring often, until vegetables soften, about 4 minutes. Mince remaining 2 cloves garlic and add. Cook, stirring, until fragrant, about 40 seconds. Add brown sugar, infused broth, coconut milk, mushrooms, and chicken. Stir to dissolve sugar, and simmer until mushroomsare tender, about 5 minutes.
4. Add lime juice and pepper and stir to mix. Taste the soup and adjust seasoning with additional salt, fish sauce, and pepper if needed. To serve, ladle into bowls and garnish with cilantro sprigs if desired.
Source: Hannaford fresh Magazine, January - February 2012