Place the beets in a large stockpot. Add 4 cups chicken broth, salt and pepper. Cover and cook for 45 minutes or until beets are tender.
Meanwhile, heat the butter in a large skillet over medium heat. Sauté the onion and carrot, constantly stirring until carrots are tender and onion is soft, and they give off a sweet aroma.
Once onion and carrots are soft, add them to the beets and broth.
Let the soup cool a little bit, then transfer to a blender, add the additional broth and blend until smooth.
Place the soup back into a pot and adjust salt and pepper to taste. Bring back to a simmer and serve.