Heat oil in a EuroCAST Stockpot over medium-high heat. Stir-fry shallot, coriander, cumin, turmeric, chiles and garlic until fragrant, about 1 minute. Add pumpkin, stock, coconut milk, lemongrass, sugar and salt, stirring to combine. Bring to a boil. Cook until the pumpkin softens, about 8-10 minutes. Quickly stir in fish sauce and lime juice.
Transfer a blender, or using an immersion blender, blend the soup to a smooth or slightly chunky consistency, as desired. Ladle into bowls and sprinkle with basil leaves. Serve immediately.