1Heat the oil in a large saucepan over medium-high heat. Add onion, bell peppers and chilli to taste. Cook, stirring, until the vegetables release some of their juices and the onion is lightly brown around the edges, 3 to 5 minutes.
2Sprinkle the vegetables with paprika, salt and cardamom and cook, stirring, until the spices are very fragrant, 1 to 2 minutes.
3Add veg broth. Stir and bring to a boil. Reduce the heat to medium-low and simmer, covered, stirring occasionally, until the peppers are fork-tender, 20 to 25 minutes. Remove from the heat and let it cool for 10 mins.
4Put the soup to a blender and puree until smooth. Return the soup to the pan.
5Whisk buttermilk and cream in a bowl ad stir into the soup.
6Serve sprinkled with basil.