Preheat your oven to 400°F (200°C) and toss the pumpkin chunks with olive oil, then spread them on a baking sheet. Roast until tender and caramelized around the edges, about 25-30 minutes, filling your kitchen with a warm, toasted aroma.
While the pumpkin roasts, heat a small amount of oil in a soup pot over medium heat. Add the minced garlic and cook just until fragrant, about 30 seconds, listening for a gentle sizzle and stirring gently so the garlic doesn't burn.
Add the roasted pumpkin to the pot, then sprinkle in the cinnamon and nutmeg. Stir to coat the pumpkin evenly with the spices, releasing fragrant, warm aromas that fill the air.
Pour in the vegetable broth, stirring to combine everything smoothly. Bring the mixture to a gentle simmer, then lower the heat and cook uncovered for about 10 minutes, allowing the flavors to meld and the soup to thicken slightly.
Use an immersion blender or transfer the soup carefully to a blender and puree until silky smooth, stopping to scrape down the sides and ensure a velvety texture. Be cautious of the hot liquid as you blend, and work in batches if needed.
Return the pureed soup to the pot and stir in coconut milk if using, to add a touch of creaminess. Warm through, tasting and adjusting seasoning with salt, pepper, and a squeeze of lemon juice if desired, to brighten the flavors.
Serve the soup hot, garnished with fresh herbs or a drizzle of olive oil if you like. The smooth, fragrant, and lightly spiced pumpkin soup is ready to enjoy, offering a comforting bowl perfect for fall evenings.