Optional Roasting:
Preheat the oven: Set your oven to 190ºC (375ºF).
Prepare the vegetables: Peel and cut the butternut squash and sweet potatoes into large chunks (about 2 inches). Slice the onion into half-moons. Place all the vegetables and garlic cloves in a roasting tin and drizzle with olive oil. Sprinkle with cumin, cinnamon, and chili powder and salt and pepper. *See notes
Roast the vegetables: Roast in the oven for about 30 minutes, until tender and golden around the edges.
Cook the soup:
If roasting, transfer the roasted vegetables to a medium saucepan. If not roasting, cut the vegetables as described and place them directly in the saucepan.
Cook the vegetables: Pour in 4–6 cups (1–1.5 litres) of water or vegetable stock over the vegetables, ensuring they are fully covered. Bring to a boil over high heat. Cook until the vegetables are completely tender—you should be able to easily insert a knife into them.
Blend the soup: Blend the soup until smooth. (See notes for blending tips.)
Add coconut milk: Stir in the coconut milk and whisk to combine. Add chili flakes and adjust the seasoning by adding more spices, salt and pepper according to taste. If needed, adjust the consistency by adding more water or stock or broth.
Serve: Swirl the reserved coconut milk on top and garnish with fresh chopped coriander.