Heat the oven to 425°F. Toss carrots with 1 tablespoon oil and 1/2 teaspoon salt on a rimmed sheet. Roast for 30 to 35 minutes, turning once, until fully tender with browned edges.
Meanwhile, heat remaining oil in a soup pot over medium heat. Cook onion for 7 minutes, then stir in ginger and garlic for 1 minute.
Add roasted carrots, broth and remaining salt. Simmer for 10 minutes, then remove from the heat and blend with an immersion blender until very smooth.
Stir in coconut milk, vinegar and white pepper. Warm gently over low heat for 3 minutes without boiling, then divide among 6 bowls.