Heat butter in a large dutch oven over medium heat. Add the sausage and the onion and cook until the sausage is no longer pink, breaking up into smaller bits as it cooks.
Add the zucchini, chicken stock, salt and pepper flakes. Bring to boil, then reduce heat to simmer and cook until the zucchini is crisp-tender.
Add the cream and heat through.
If you wish to thicken the soup, mix the flour and water and gradually stir into the soup. Heat until the soup gently bubbles then cook, stirring frequently, until the flour is cooked--about 5 minutes.
For UK and EU--"half and half" is equal parts of whole (full fat) milk and single (light) cream.