Preheat the oven to 400°F. Place squash, apple, and onion cut-side down on a parchment paper lined baking sheet. Roast for 30 minutes. Transfer the apple and onion to a blender. Return the squash to the oven. Roast for 15 to 30 minutes more or until tender. Let cool slightly.
Scoop the flesh from one butternut squash half into the blender with the apple and onion. Add 2 cups of the broth, the curry powder, and the salt. Puree until smooth. Transfer to a large saucepan. Add the remaining broth, and bring to a boil. Reduce heat and simmer, covered, 10 minutes. Serve topped with yogurt, cilantro, and, if using, pumpkin seeds.
Scoop the flesh from the remaining squash half into a bowl and roughly mash. Transfer 1 cup portions to air-tight containers and freeze to use in Butternut Squash Ravioli.