In a 4-quart Dutch oven, combine chicken broth, beef brisket or franks, sauerkraut, carrot, onion, garlic, thyme, white pepper, tarragon, and bay leaf. Bring to boiling; reduce the heat. Cover and simmer for 30 minutes.
Remove bay leaf. Stir together the cold water and cornstarch. Stir cornstarch mixture into soup. Return to boiling. Cook the soup for 2 minutes.
Reduce heat. Stir in the Swiss cheeses until melted. Stir in cream. Heat through. Serve topped with croutons. Makes 6 to 8 main-dish servings.