In a pot, add the broths, onion, celery, bell pepper, and potato. Place on high heat and bring to a boil, then reduce heat to a simmer and cook until the potatoes are just tender, about 10 to 15 minutes.
Mix together the cornstarch and cold water, stir that into the soup, and bring the soup to a boil while stirring, then reduce to a simmer and cook until the soup thickens.
While the soup is simmering, cut the sausage into quarters lengthwise, then crosswise into 1/4 inch pieces.
Add the sausage, Swiss cheese, sauerkraut, cream, and milk to the soup and stir everything together. Heat on a low simmer until heated through, do not let the soup boil.
When it is hot, remove from heat, stir in the white pepper.
Ladle into bowls and garnish with spring onion. Serve and enjoy.