Cut the dry, woody tops from the lemongrass stalks. Smash the remaining "fleshy" part with the back of your knife to break it apart slightly. Cut into ½ - 1 inch pieces.
In a large pot bring chicken stock, lemongrass, ginger, kaffir lime leaves (or lime zest), and Thai red chilis to a boil. Reduce heat to a gentle simmer. Cover and simmer gently for 20-30 minutes to infuse the stock.
Strain stock and set aside. Discard aromatics.
Heat oil in the same pot over medium high heat. Add onions and sauté for 3-4 minutes or until starting to soften.
Add strained stock to onions. Stir in coconut milk, chicken, mushrooms, green peas, fish sauce and brown sugar.
Cover and continue simmering for 10 minutes or until chicken is cooked through.
Remove from heat and stir in fresh lime juice and cilantro.
Ladle into bowls and garnish with thinly sliced red onion, red pepper and extra lime juice.