In a large pot, heat a drizzle of olive oil over medium heat. Add diced chicken, season with salt and pepper, and sauté until browned, about 5–7 minutes. Once cooked through, remove the chicken from the pot and set it aside on a plate, leaving any drippings behind for flavor.

In the same pot, toss in chopped onions, carrots, and celery. Sauté the mixture over medium heat for about 5–7 minutes, or until the vegetables soften and onions become translucent.

Stir in 2 tablespoons of tomato paste, 1 tablespoon of Worcestershire sauce, and 1 tablespoon of Dijon mustard. Allow the mixture to cook for about 1 minute, stirring constantly.

Return the cooked chicken to the pot and add in 1 cup of rinsed sauerkraut and 4 cups of beef broth. Raise the heat and bring the mixture to a boil. Once boiling, reduce the heat to low and let it simmer for 15 minutes.

Sprinkle in 1 tablespoon of agar-agar powder while stirring, and let the soup simmer for an additional 2–3 minutes. Remove the pot from heat, then stir in 1 cup of heavy cream and 1 cup of shredded Swiss cheese until smooth and creamy.

Taste the soup and adjust the seasoning with salt and pepper to your liking. Serve the warm Creamy Reuben Soup in bowls, garnishing with extra Swiss cheese on top, if desired.