Juice the oranges.
Dry-fry the spices in a saucepan over a medium heat for 1–2 minutes. Add the onion, carrots, red lentils, orange juice and vegetable stock, bring to the boil and simmer for 25–30 minutes until the carrots are tender and the lentils are soft.
Transfer the soup in batches to a food processor or blender and process until smooth, transferring each successive batch to a clean saucepan. Heat through gently.
Ladle the soup into bowls, top with a swirl of yogurt and garnish with coriander sprigs.