In a 4- to 6-quart heavy pot, toast cumin seeds over medium-low heat for 30 to 60 seconds, shaking the pan occasionally so seeds toast evenly. Watch closely so seeds don’t burn. Transfer seeds to a spice grinder (or clean coffee grinder). Cool completely. Grind until finely crushed.
In the same pot, cook onion in hot oil over medium-low heat for 6 to 8 minutes or until lightly browned, stirring occasionally.
Meanwhile, trim and peel squash. Cut squash in half to reveal the seeds; scoop out seeds and roast for a garnish if desired**. Cut squash into about 1-inch pieces. Add squash, broth, apple, and salt to pot with onion. Bring to boiling; reduce heat. Simmer, covered, for 20 to 25 minutes or until squash and apple are tender. Stir in crushed cumin.
Cool soup slightly. Use a blender or immersion blender to blend soup until completely smooth. Return soup to the pot. Heat through over medium-low heat. Season with additional coarse salt if desired.
To serve, ladle soup into serving bowls. Top with freshly ground pepper and roasted squash seeds (if using).