Place a large pot over medium heat and spray with olive oil. Cook onion, celery and garlic and cook for 1-2 minutes, or until just softened.
Add the spices to the pan and cook for another 1-2 minutes until fragrant.
Add sweet potato, tomatoes, lentils, vegetable stock and water to the pot and bring to the boil. Reduce heat and simmer, partially covered, for 20-25 minutes or until sweet potato is tender, cooked through and soup has thickened. Stir through spinach.
Divide the soup between 4 bowls and serve with chopped parsley, cracked pepper and grainy bread.