Preheat oven to 400°F and toss cubed butternut squash with olive oil, salt, and pepper on a baking sheet.
Roast the squash for 30-35 minutes until tender and lightly caramelized.
Meanwhile, sauté chopped onion in a large pot over medium heat for 5 minutes, then add garlic and cook for 1 minute.
Add the roasted squash, vegetable broth, cinnamon, and nutmeg to the pot and bring to a simmer for 10 minutes.
Use an immersion blender to puree until silky smooth, then stir in maple syrup.
Taste and adjust seasoning, then serve hot with toasted pepitas if desired.