Step 1 Place a large saucepan over medium heat and spray with oil. Cook onion and celery, stirring, for 6–7 minutes, or until softened. Add garlic and oregano and cook, stirring, for 1 minute.
Step 2 Add sweet potato, cook for 1 minute, then add stock, 1 cup water, lentils, chickpeas and tomatoes and bring to the boil. Reduce heat and simmer, partially covered, for 15–20 minutes, or until sweet potato is tender.
Step 3 Add beans and simmer for 3–4 minutes, or until beans are tender-crisp. Divide soup between 4 serving bowls. Serve with a tablespoon of yoghurt, chopped parsley and toast.