Preheat oven to 375 degrees F.
Cut squash in half and remove seeds. Brush with oil and place cut-side down in a baking pan. Roast until tender, about 45 minutes. Let stand until cool enough to handle, then scoop the squash out of the skin.
Meanwhile, place curry powder and cumin in a small saucepan over medium-low heat; stir until fragrant, 2 to 4 minutes. Add coconut milk and ginger; bring to a boil over high heat. Adjust heat to maintain a simmer and simmer, stirring frequently, until reduced to about ¾ cup, 13 to 15 minutes.
Process pepitas in a food processor until finely ground. Add the squash, the coconut milk mixture, lime zest, lime juice and salt; process, scraping down the sides once or twice, until smooth.
Refrigerate until cold, about 2 hours. Serve sprinkled with cilantro and pepitas, if desired.
