Put the oil in a large saucepan over medium heat. Add the onion; cook and stir for 5 minutes or until tender. Add the farro and cook and stir for 2 minutes or until toasted.
Using a ladle or measuring cup, begin adding broth to the farro, about 1/2 cup at a time. Cook and stir after each addition until liquid is absorbed. Continue adding broth, stirring frequently, until the farro is tender but not mushy (should take about 40 minutes total).
Stir in the butter, Parmesan, and peas. Cook and stir to heat through. Serve immediately.