In large saucepan, heat oil over medium high heat. Add onion, celery and carrot, salt and pepper. Cook until tender, about 5 minutes.
Pour in broth and bring to boil. Add sweet potato and reduce heat to maintain a simmer. Cover and cook until sweet potatoes are tender and crisp, about 10 minutes. Stir in kale and cook 10 minutes more. Let cool slightly.
Use a ladle to transfer soup to a blender in batches, processing until smooth. Repeat with remaining soup. Transfer back to saucepan and thin with water if desired.
Serve warm sprinkled with wholegrain croutons.
Per serving: about 190 calories, 4 g protein, 3 g fat (0 g saturated fat), 38 g carbohydrate, 7 g fibre, 410 mg sodium. %RDI: 15% calcium, 10% iron, 590% vitamin A, 100% vitamin C.