Heat oil in a pot set over medium heat. Add onion, garlic, ginger and carrots and cook until vegetables are slightly softened, about 5 minutes. Mix in curry powder and flour. While stirring, slowly pour in stock and orange juice and bring to a simmer. Cook until carrots are tender, about 20 minutes. Puree mixture in a blender or food processor. Return soup to the pot and bring back to a simmer. Season soup to taste. Pour into bowls, garnish with a spoon of yogurt and parsley sprig, and serve.