In Thailand, kabocha squash is popular in many restaurant and home kitchens. Here, it can still be a little hard to find, so this recipe was created with butternut squash and the flavors of Thai cuisine in mind, but easily accessible ingredients. It must be said, however, that this is a thick soup. If you prefer a thinner soup, add one to two tablespoons of warm water or broth to each portion before serving.
Preheat the oven to 400 degrees.
Coat a large baking sheet with cooking spray. Place the squash in an even layer on the baking sheet and roast, stirring after 20 minutes, until browned and easily pierced with a knife, about 40 minutes. Remove from the oven and let cool.
Meanwhile, heat the oil in a large pot over medium heat. Add the onions and cook until lightly browned, stirring occasionally, about 10 minutes. Add the garlic and ginger. Cook, stirring, until fragrant, about 1 minute. Add the curry powder. Stir to combine and then cook for 30 more seconds.
Add the roasted squash to the pot and stir to combine. Add the broth, coconut milk, salt, and cayenne. Cover and simmer for 10 minutes.
Purée the soup in batches in a blender or food processor or in the pot using an immersion blender (be careful of splattering liquid). Before serving, stir in the lime juice. Ladle the soup into bowls and garnish with the scallions and cilantro.