Heat 1 tablespoon oil in a large pot over medium heat. Add 1/3 cup onion, 1/4 cup celery, and finely chopped ginger and sauté until the onion is tender and translucent, about 8 minutes.
Stir in 2 cups vegetable broth, carrots, potatoes, and 3/4 teaspoons curry powder and bring to a boil. Season everything with salt. Decrease the heat to maintain a simmer. Cook, stirring occasionally, until the carrots are tender, about 25 minutes.
Using an immersion blender (or using a regular blender and working in batches), blend until completely smooth. Return the soup to the pot over low heat and cook, stirring often, until heated through.
Serve the soup hot and garnish with 1 tablespoon crème fraîche. Drag a butter knife or toothpick through the swirls for a marble effect.