Preheat the oven to 400º F.  Make the stock and keep it warm over low heat.  Place the squash, cut side down, in a baking dish with a little water. Cover and roast until tender, 35 to 40 minutes. When the squash is cool enough to handle, scoop it out of the skin. You should have about 4 cups.  Heat the oil in a soup pot and add the onions, ½ teaspoon salt, and a pinch of pepper and cook until they begin to soften, about 3 minutes. Add the garlic, lemongrass, and ginger and cook for 2 minutes. Add the squash, the stock, and the lime leaf and bring to a boil. Lower the heat and simmer, uncovered, about 30 minutes.  Remove the lime leaf and puree the squash mixture in a blender until smooth. Pass through a food mill and return to the pot over medium-low heat. Add the coconut milk and cook for 5 to 10 minutes. Adjust the seasoning with salt and pepper to taste. Garnish each serving with Thai basil.