Heat the oven to 425°F. Toss the squash cubes, onion wedges, and whole garlic cloves with the olive oil, salt, and pepper.
Spread everything on a large sheet pan in a single layer with space between the pieces — crowd the pan and it steams instead of browning.
Roast for 40 to 45 minutes, flipping once halfway, until the squash is fork-tender and the edges are deeply golden and caramelized.
Transfer the roasted vegetables to a blender along with 2 cups of the warm broth. Blend until completely smooth, working in two batches if needed.
Pour the puree into a large pot and stir in the remaining broth a little at a time until it reaches the thickness you want.
Add the nutmeg and cayenne and bring to a gentle simmer over medium-low heat.
Stir in the half-and-half and the maple syrup if using. Warm through without letting it boil, then taste and adjust the salt.
Serve topped with toasted pumpkin seeds and a swirl of cream.