Heat oil in a large saucepan or stockpot over medium heat. Add onion, celery, carrot and garlic. Cook for 10 minutes, stirring occasionally, until vegetables are softened slightly.
Add curry powder and lentils. Stir over heat a further minute.
Add passata, stock cube and 6 cups water. Bring to the boil. Reduce heat. Simmer for 30 minutes, skimming surface occasionally to remove any foam.
Stir in zucchini and parsley. Simmer a further 5 minutes until vegetables and lentils are tender.
Meanwhile, to make toasts, place bread rolls, cut-side up on a baking tray. Brush with oil. Sprinkle with parmesan. Bake in 200C oven (fan-forced) for 5-8 minutes until crisp. Serve with soup.

Leftover soup can be kept in covered in the fridge for up to 5 days. Reheat in a saucepan or in a microwave-safe bowl in microwave, adding a little water to thin consistency if needed.
Lentil soup mix is available from the soup aisle in supermarkets. It consists of red split lentils, French style (puy) lentils, green lentils and yellow split lentils.