Warm the olive oil in a large pot over medium heat.
Sauté the onion until softened and translucent, about 4–5 minutes.
Stir in the garlic and ginger; cook until fragrant, about 30 seconds.
Add the carrots and chili-garlic sauce; cook, stirring, 2 minutes.
Deglaze with soy sauce and rice vinegar; stir for 30 seconds.
Pour in the vegetable broth. Bring to a boil, then simmer covered until carrots are very tender, 15–18 minutes.
Ladle about 1 cup hot liquid into a bowl and whisk in the miso until smooth.
Blend the soup until silky using an immersion blender, or carefully blend in batches.
Return to low heat. Stir in the miso mixture and coconut milk; warm gently 2–3 minutes without boiling.
Finish with lime juice and sesame oil. Season with salt and pepper to taste.
Ladle into bowls and top with scallions and sesame seeds. Serve hot.