There are several ways to thicken broccoli and cheddar soup. One option is to use a thickener such as flour, cornstarch, or potato starch. To do this, mix a small amount of the thickener with a little bit of cold water to form a slurry. Slowly stir the slurry into the soup, then bring the soup to a simmer and cook for a few minutes until it has reached your desired consistency.