Heat a large saucepan (big enough to hold all the ingredients) over a medium heat. Pour in a good lug of oil then add the onions, leek, celery, peppers, courgettes, carrots and herbs.
Cook the whole lot slowly for about 20 minutes with the lid on, until all the vegetables are nice and soft but without any colour at all. Add the tomatoes, squash, 500ml of water and a pinch of salt and pepper to the vegetables.
Bring to the boil and simmer gently for about 30 minutes until the squash is soft.
Fish out the bay leaves and allow the sauce to cool slightly before blitzing with a hand-held blender until nice and smooth.
Have a taste and season with a little more salt and pepper if needed.