On a steady work surface, using a mallet or the back of a sturdy knife, bruise the root end of the lemongrass. In a large pot over high heat, add the lemongrass, vegetable stock, and ginger and bring to a boil. Reduce the heat and simmer for 15 to 20 minutes. Strain the stock and return it to the pot. Add the butternut squash to the stock and simmer until the squash is fully cooked and begins to break down, about 1 hour. Remove from the heat and cool. Add the coconut milk, blending the mixture in batches until smooth. Season with lime juice and salt, and chill. To serve, garnish the soup with cilantro, cashews and coconut.