Preheat the oven. Arrange a rack in the middle of the oven. Heat the oven to 425ºF/218ºC.
Cut and seed the butternut squash. Slice the ends off the butternut squash to create a flat surface. Stand the butternut squash on the bulb end and slice through it lengthwise. Use a large spoon to scoop out the seeds.
Roast the butternut squash. Drizzle the butternut squash with a little bit of olive oil, season it with salt, and place it cut-side down onto a baking sheet (you can line the baking sheet with parchment paper for an easier clean up). Roast the butternut squash until tender, for 40-50 minutes, depending on the size of the squash.
Roast the aromatics. At the 20-minute mark, lightly coat the onion with olive oil and place it cut-side down onto the baking sheet together with the unpeeled garlic cloves*. Continue roasting the butternut squash with the aromatics.
Blend. Scoop out the butternut squash flesh into a high-speed blender. Then add the roasted aromatics, coconut milk, ginger, lime juice, vegetable broth, and salt, and blend until completely smooth. If using a Vitamix blender, blend until heavy steam escapes from the vented lid, 5-6 minutes.
Season. Taste and adjust the flavor as needed, adding more salt for saltiness, lime juice for acidity, and ginger for a spicy bite. If the soup is not sweet enough, you can add a little bit of maple syrup.
Serve. Pour the soup into serving bowls - it will be hot straight out of the blender (if using Vitamix). Garnish with a drizzle of coconut milk and toasted butternut squash seeds.
Store. Leftover soup keeps well, covered, in the refrigerator for 3-4 days. For longer term storage, freeze in an airtight container for up to 3 months. Reheat. Transfer frozen soup into the refrigerator 24 hours before reheating to thaw slightly. Reheat in a pot on the stovetop over medium-low heat, stirring frequently, until hot. Add ¼ cup/60 ml water/broth at a time if too thick.