Heat the olive oil in a large pot over medium-low heat. Add the ancho and cook, turning, until toasted, about 1 minute. Add the shallots and cook, stirring occasionally, until softened, about 3 minutes. Add the squash, cumin and 1 teaspoon salt, stirring to coat. Add 6 cups water and bring to a boil; reduce the heat to medium low and simmer until the squash is tender, 25 to 30 minutes.
Working in batches, transfer the soup to a blender and puree until smooth (or puree in the pot with an immersion blender). Return the soup to the pot; stir in the agave and lime juice. Season with salt and thin with water as needed. Top each serving with pepitas and sprinkle with cayenne.
Photograph by Levi Brown
You this soup up to 2 weeks ahead and freeze in an airtight container.