Preheat your oven to 200°C (390°F). Spread the peeled and chopped sweet potatoes and carrots on a baking sheet, toss with a little olive oil, and roast for 25-30 minutes until tender and slightly caramelized.
While the vegetables are roasting, peel and dice the onion. Sauté it in a large pot over medium heat with a splash of oil until translucent and fragrant, about 5 minutes.
Add the minced garlic and grated ginger to the sautéed onion, cooking for another minute until fragrant and slightly bubbling.
Remove the roasted vegetables from the oven once tender and slightly caramelized, then add them to the pot with the sautéed aromatics.
Pour in the vegetable broth and bring everything to a gentle simmer. Let it cook for about 10 minutes, allowing the flavors to meld.
Use an immersion blender directly in the pot, or carefully transfer the soup to a blender, blending until silky smooth and velvety. Be cautious of hot splashes if using a regular blender.
Return the blended soup to low heat, stir in the coconut milk for creaminess, and season with salt, pepper, and a squeeze of lemon juice to brighten the flavors.
Taste the soup and adjust the seasoning if needed. Serve hot, garnished with seeds or herbs if desired.