Heat the oil in a pot over medium heat. Add carrots and onion and cook until slightly softened, about 5 minutes. Mix in curry paste and flour. While stirring, slowly pour in
stock. Bring to a boil, and then adjust heat so the soup gently simmers. Cook until carrots are tender, about 20 minutes. Working in batches, puree mixture in a blender or food processor. Return soup to the pot and bring back to a simmer. Stir in the cilantro; season with salt and pepper.
Pour soup into bowls. If desired, set a spoonful of yogurt in the centre of each bowl.