Heat the oil in a large pan over a low heat. Add the red pepper and celery and cook for about 10 minutes stirring occasionally, until softened and lightly golden.
Stir in the tomatoes and sugar. Season with a pinch of salt and plenty of pepper. Bring to a simmer and cook gently for 15–20 minutes, stirring occasionally, until the flavours are well combined. Tear in the basil and then blitz the soup with a hand-held blender.
Ladle the soup into warmed bowls and add a sprig of fresh basil to each one to serve.