Preheat the oven to 400 degrees. Lightly oil a large baking sheet. Place the halved yams, squash and onions cut side down on the baking sheet. Bake for 50 minutes or until the vegetables are soft. Remove from the oven and allow to cool. Scoop the flesh out of the yam and squash skins. Cut the roasted onion into large chunks.While the vegetables are roasting, heat the oil in a large pot and fry the spices, stirring, for about 1 minute, or until fragrant. Add the roasted vegetables, stock, coconut milk and minced ginger (if using) and simmer until heated through. Puree the soup in a blender or with an immersion blender until smooth. Add more water if needed.Serve garnished with chopped parsely.IMPORTANT! Always use great caution when pureeing hot liquids. If you have time, allow the soup to cool before blending. Never fill the container more than half full and cover the lid with a folded kitchen towel and hold it in place before you turn on the blender. The initial surge of the blender's contents can cause the lid to pop open, possibly scalding you if still hot and certainly making a mess of your kitchen.