Replace cornflour in soup
Cook the flour with some butter or oil before adding to the soup. You are making what is called a roux which is a traditional French method for thickening sauces and soups

Measure roughly two parts of general purpose flour and one part of fat (or equal parts by weight), and cook until bubbling and the raw flour taste has gone, or it is lightly brown

Approximately one teaspoon of flour per litre of soup

The other option is to use arrowroot, which is an alternative starch to cornflour