In a large saucepan over medium-high, heat the oil until shimmering. Add the onion, celery, rosemary, ¼ teaspoon salt and ½ teaspoon pepper. Reduce to medium and cook, stirring often, until the vegetables begin to release their moisture, about 2 minutes. Reduce to medium-low, cover and cook, stirring occasionally, until the vegetables are softened but not browned, 10 to 15 minutes.
Add the tomato paste and cook over medium-high, stirring, until the paste browns and slightly sticks to the pan, about 2 minutes. Add the farro and stir until the grains are well combined with the paste. Add the Parmesan rind (if using) and the broth. Bring to a simmer, then reduce to medium and simmer, uncovered, stirring once or twice, until the farro is tender with a little chew, 25 to 30 minutes.
Off heat, remove and discard the Parmesan rind (if used). Taste and season with salt and pepper. Serve sprinkled with the Parmesan and a generous grind of pepper.
