Cut the corn off the cob and place it in a large bowl; transfer about 75g of the corn into a cup and set aside for adding to the soup after it has been pureed.
Heat oil in a saucepan over medium heat; add onion and cook until softened. Add the carrots and celery; continue to cook for another 5 minutes. Add the corn and red pepper and pour in enough water to cover the vegetables.
Bring to the boil; reduce the heat and let it simmer for 15 minutes.
Blend the soup until smooth in a blender or food processor; transfer back into the pot and stir in reserved sweetcorn.
Serve the soup drizzled with harissa oil and fresh basil leaves.
You can use homemade roasted red peppers if you prefer but best to peel them before using in this soup.