Heat oil in a large saucepan over medium-high heat. Add onion and garlic. Cook, stirring often, for 3 minutes. Stir in coriander, cumin and chilli powder. Cook, stirring, for 1 minute. Add sweet potato and carrot. Cook, stirring often, for 5 minutes. Add stock. Cover. Bring to the boil. Reduce heat to medium-low and simmer, stirring occasionally, for 20 minutes.
Add chickpeas to soup and simmer, covered, for 10 minutes or until chickpeas are tender.
Blend soup, in batches, until smooth. Return to saucepan over medium-low heat. Season with salt and pepper. Stir in 1 tablespoon lemon juice. Heat, stirring, until hot (do not boil). Ladle into bowls. Top with croutons. Sprinkle with pepper. Serve.