Put the garlic, celery and red onion in the slow cooker. Add the tomatoes, bay leaves, parsley, sugar and stock, then season with salt and pepper.
Cover the cooker with the lid and cook on HIGH for 2½ hours. Stir the tomatoes well. Switch the cooker to LOW, re-cover and cook for a further 5½ hours until the tomatoes have broken down and the vegetables are tender. Remove and discard the