The best pattern to thicken French onion soup is by adding a roux or a mixture of melted butter and flour that is cooked until it reaches a paste-like consistency. You can also add some cornstarch or arrowroot powder to the soup for additional thickening power. To use these ingredients, mix them with cold water until you get a smooth liquid and stir it into your boiling soup. Be sure to simmer the soup for 15 minutes more so the thickener has time to fully integrate before serving. Another choice is to puree some of the cooked onions in a blender and then add them back into the finished soup for added thickness. This method will give your soup an even creamier texture. Finally, if you're looking for a quick and easy way to thicken French onion soup, you can simply add more cheese to the soup. The melted cheese will act as a natural thickener while also adding an extra layer of flavor. Enjoy!