Step 1 Melt the butter in a stockpot over medium heat. Add the onion, celery and diced fennel and cook, stirring constantly, for 1 minute. Cover and cook until the vegetables are softened but not brown, about 5 minutes. Add the coriander, cumin, caraway seeds and pepper. Cook, stirring, until the spices are fragrant, about 1 minute. Add the beets, cabbage and broth and bring to a simmer. Partially cover the pot and simmer until the beets are tender, about 35 minutes.
Step 2 Add the cubed beef to the pot and heat through. Add salt to taste. Ladle into warm soup bowls. Serve with a dollop of sour cream and garnish with the chopped fennel fronds.