Bring farro, 3 cups water and ¼ teaspoon salt to a boil. Reduce heat to medium-low, cover and simmer 25 minutes, or until grains are al dente. Drain excess liquid. Heat oil and butter in a large saucepan over medium heat. Add onions and carrots and cook until soft, about 5 minutes. Add garlic and heat 1 minute. Add fennel, red chili flakes, remaining salt and black pepper and heat 30 seconds. 

Add wine, bring to a boil and simmer 3 minutes. Add tomatoes, 1 cup water and thyme. Bring again to a boil, then add mussels. Reduce heat to medium, cover and cook 3 minutes, or until shells pop open. 

Use a slotted spoon to remove mussels from soup; discard any that have not opened. Stir farro, red wine vinegar and olives (if using) into soup. Remove mussels from shells and add flesh and their juices to soup. Serve garnished with parsley.