In a large pot, melt the unsalted butter over medium heat. Add chopped onion and minced garlic, sautéing for about 5 minutes until the onion is translucent and fragrant.

Sprinkle in the all-purpose flour and stir continuously for 2 minutes until the roux is slightly golden, ensuring it doesn't brown too much.

Gradually pour in the chicken broth while whisking to prevent lumps. Increase the heat a little and bring the mixture to a gentle simmer, about 5 minutes.

Once simmering, lower the heat and stir in the heavy cream, mixing thoroughly until well combined.

Mix in the chopped corned beef and drained sauerkraut, stirring until fully incorporated and simmer for an additional 5 minutes.

Add the shredded Swiss cheese and Russian or Thousand Island dressing to the pot. Stir continuously until the cheese is melted and fully integrated.

Season the soup with salt and pepper to taste, and let it simmer for another 10 minutes on low heat.

Ladle the hot soup into bowls and garnish each serving with rye bread croutons and a sprinkle of chopped parsley.