Using a mortar and pestle, or similar, bash the garlic, chilli and lemongrass so they burst open to release flavour then add into a soup pot.
Add the kaffir lime leaves, galangal, chicken stock and water to the pot. Simmer on high heat, cover, then reduce to medium heat and simmer for 10 minutes.
Strain the broth, discard the aromatics, then return broth into same pot over low heat.
Add onions, mushrooms, tomatoes, shrimp and simmer 2 minutes or until just cooked. Next stir in sugar, fish sauce, lime juice, and coconut milk. Adjust the seasonings to your liking.
Ladle into bowls and serve with fresh cilantro and more Thai chili!