It turns out, different soups go well with different levels of complexity of this sandwich. The trick is to find a soup that has the same level of flavor as the sandwich.For example, if you're eating a Reuben Sandwich from a deli, you'll want to play it safe with an onion soup.If you're home-cooking your Reuben and have some world seasoning in your kitchen, twist it up and try something bolder like Matzo Ball soup or the Asian-inspired Tomato Soup.Just make sure it has similar intensity of flavor so it doesn't clash with the sandwich.