Heat the oil in a soup pot over medium heat and add shallots, lemongrass, Garlic, and chili paste. Cook until aromatic, about 30 seconds.

Add the galangal, lime leaves, stock, sugar, and coconut milk. Bring to a boil, reduce heat, and simmer for 15 minutes.

Strain the broth into a pot and discard the solids. Add the chicken, mushrooms, and tomatoes. Simmer for 25 minutes.

Add the lime juice and salt and adjust seasoning with sugar and salt. Garnish the soup with cilantro.