Preheat the oven to 425°F and line a baking sheet with parchment paper.

Place the butternut squash on the baking sheet, cut side up, with the onion.  Cut 1/2-inch off the garlic, drizzle with oil and wrap in foil.  

Brush the flesh of the squash and the onions with oil and sprinkle with salt and pepper.  Transfer to the oven and roast until the vegetables are tender, about 45-50 minutes.  Remove from the oven and allow to cool. 

When the vegetables are cool enough to handle, spoon the squash into a blender bowl along with the onions.  Unwrap the garlic and pick out the cloves and place them into the blender bowl.  Add 2 cups of the chicken broth and turn the machine on to low, gradually increasing the speed.

Process the vegetables to a thick purée adding additional chicken broth if needed.  When smooth and creamy, remove the container from the machine and reserve.

Heat the remaining 2 teaspoons of oil in a 6-quart Dutch oven or heavy bottomed pan, set over medium heat. Add the ginger, lemongrass and red curry paste, sauté for 1-2 minutes stirring frequently. 

Pour in the puréed vegetables and stir in the remaining chicken broth, chili-garlic sauce, fish sauce, lime juice and the kaffir lime leaves, if using.  Bring the soup to a low boil, then reduce the heat to low, stir in the coconut cream and simmer for 10 minutes.  

Remove and discard the kaffir lime leaves and ladle the soup into bowls.  Serve with lime wedges and garnish each serving with Thai basil leaves, chopped peanuts and a swirl of hot chili finishing oil.