Martha Stewart reveals a secret tip for thickening tomato soup without using heavy cream. Instead, she uses a roux made of butter and flour. The roux adds body and creaminess to the soup without the need for heavy cream. It can be added at the beginning or end of the recipe. Making a roux takes a little more effort but can be as simple as melting butter, adding flour, and stirring constantly. This alternative thickener allows for a smooth texture and longer storage time.