Peel the outer leaves of the lemongrass stalk until you get to the tender center. Slice the lemongrass thinly. Set aside.Heat the oil in a heavy-bottomed pot over medium-low heat. Add the onions and garlic and let them cook for about 4 minutes, until the onions turn translucent. Make sure that the heat isn't too high so that the onions and garlic don't burn. Add the ginger and lemongrass and cook for another minute.Add the vegetable broth, Bring heat up to a medium-high. Once the stock to boils, add butternut squash, soy milk, salt and turmeric (if you're using it), and bring soup to boil again. Add lime juice and give everything a good stir. Turn off the heat.Using an immersion blender, blend the soup until smooth. Alternatively, you can scoop everything into a blender and blitz until smooth.Serve soup with coconut milk, toasted coconut, pumpkin seeds, cilantro, and green onions if you like.