Mustard greens have a sharp, radishy bite that's delicious in a bean soup, but, if you like, you can substitute kale or collards. Smaller white beans such as cannellini can be used in place of giant white beans.
Place beans in a large bowl, cover with 2 inches cold water; let soak overnight at room temperature. Drain and rinse beans. (Or quick-soak them: Boil beans 1 minute, then let them sit off heat, covered, 1 hour.)
Preheat oven to 300 degrees. Heat oil in a large, heavy-bottomed ovenproof pot over medium-high heat. Add onion and carrots, and cook, stirring occasionally, until vegetables are golden brown in places, 8 to 10 minutes. Add garlic and greens to pot, and cook, stirring frequently, until greens wilt, about 2 minutes.
Tie together thyme, parsley, and bay leaves with kitchen twine, and add to pot along with beans, Parmesan rind, and water. Bring to a boil over high heat. Cover, transfer to oven, and braise 1 hour.
Remove from oven, and stir in 1 tablespoon salt and 1/2 teaspoon pepper. Cover, return to oven, and braise until beans are tender, about 30 minutes. Season with salt and pepper.
Meanwhile, bring a large pot of salted water to boil. Add farro, and cook until grains are tender and begin to split, about 25 minutes. Drain.
Divide farro among 6 bowls, and ladle soup over farro. Sprinkle with Parmesan, and drizzle with oil. Soup and farro can be refrigerated in separate containers 2 days; let cool completely before refrigerating. Gently reheat together in a covered pot over medium heat, stirring occasionally.