Heat the oil in a large saucepan over medium heat. Add the onion, 1/2 teaspoon salt, and 1/4 teaspoon pepper and cook, stirring occasionally, until soft, 5 to 6 minutes. Add the farro and wine and cook, stirring, until the wine is absorbed.
Stir in the broth, 1 cup at a time (waiting until each addition of broth is absorbed before adding the next), and cook, stirring occasionally, 20 to 25 minutes total.
Stir in the arugula and Parmesan.