Melt butter in a large pot over medium heat. Add onion and garlic; cook and stir until softened, about 5 minutes.
Add broth, tomatoes, Worcestershire sauce, mustard, hot sauce, caraway seeds, and black pepper. Bring to a boil, reduce heat to low, and simmer until flavors have blended, about 10 minutes.
Stir in corned beef and continue to simmer until heated through, about 5 minutes.
In a small bowl, whisk together sour cream and flour until smooth. Stir into the soup and simmer until thickened, about 5 minutes.
Stir in parsley and season with salt, if desired. Ladle soup into bowls and top with Swiss cheese and croutons.
Interesting Facts
The Reuben sandwich is an American classic, made with corned beef, Swiss cheese, sauerkraut, and Russian dressing on rye bread.
This soup is a great way to use up leftover corned beef.
You can use any type of cheese you like, such as cheddar or Gouda, in place of the Swiss cheese.