Saute meat in oil until well browned. Add onion and carrot, saute for 10 minutes. Add broth and beans. Simmer for 40 minutes. Add tomato paste, rice and potato. Simmer for 15 minutes. Add salt and pepper. Simmer for 5 minutes. Ladle soup into bowls and sprinkle with scallions and dollop of yogurt if using. Serve.