Step 1Melt 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 2Add 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.