Trim and halve lengthwise carrots and cut into chunks. Cut sweet potato into medium-size chunks. (You don't have to peel either if you prefer.) Toss with 1 tablespoon sweet potato and spread out on a rimmed baking sheet. Roast for 20 minutes.
In a large stockpot, heat remaining oil over medium heat. Add onion and celery and sauté 5 minutes. Add the carrots and sweet potato.
Stir in broth and bring to a boil, then let simmer for 15 minutes.
Using an immersion blender (or blend in batches with a food processor or blender) to purée soup smooth.
Whisk in miso to taste and keep warm while making the sauce.
In a blender or food processor, purée cilantro, garlic, cream, and water until smooth. Add more water if needed to achieve a pourable, yet still thick, consistency.