Pour the broth in a large saucepan, bring to boil on high, add garlic, lemongrass and 3/4 of chili. Reduce the heat to medium, add chicken breast and simmer for 10-15 minutes.
Take out chicken from the broth and cut into thin strips.
Bring soup to boil, add fish sauce, sugar, lime juice and remaining chili pepper. Simmer for 5 minutes.
Pour cooked soup in bowls, add chicken meat and garnish with coriander leaves.
Ready in: 40 minutes.