Melt the butter in a large saucepan over a medium heat. Cook the onions and celery for 5 minutes, stirring occasionally, until starting to soften. Stir in the tomatoes, squash and stock and bring to the boil. Turn down the heat and simmer for 30-35 minutes, until the squash is really tender. Cool.
2. Either blitz in the pan with a stick blender or whizz in batches in a food processor or blender, until smooth. Gently reheat, stirring in the cream, until just simmering. Remove from the heat and season to taste.