In a large pot set over medium heat, melt 4 tablespoons of unsalted butter until bubbling. Add 1 diced onion and 2 minced cloves of garlic, cooking for about 5 minutes until the onion turns translucent and fragrant.

Once the aromatics are ready, sprinkle in 1/4 cup of all-purpose flour, combining it thoroughly. Keep stirring for about 2 minutes to cook the flour and form a roux.

Gradually whisk in 4 cups of chicken broth, ensuring you eliminate any lumps. Raise the heat slightly and bring the mixture to a gentle simmer, about 5 minutes.

Lower the heat to medium-low, then pour in 1 cup of heavy cream while stirring to combine. Allow the mixture to heat through for another 3-4 minutes.

Stir in 2 cups of chopped corned beef and 1 cup of well-drained sauerkraut. Continue cooking for about 5 minutes.

Add 1 cup of shredded Swiss cheese and 1/2 cup of Russian or Thousand Island dressing into the pot. Stir until the cheese is completely melted and creamy.

Taste your soup and add salt and pepper according to your preference. Allow it to simmer gently for an additional 10 minutes.

Ladle the soup into bowls while it's still hot, garnishing with crispy rye bread croutons and a sprinkle of chopped parsley.