Heat oil in a Dutch oven over medium-high heat. Add onion, mushrooms, salt and pepper; cook 8 to 10 minutes, stirring frequently, until onions are softened and most of liquid from mushrooms has evaporated. Heat broth in a large microwave-safe measuring cup or bowl until hot, about 3 minutes on HIGH; set aside.
Add farro, garlic and rosemary to mushrooms; cook 2 minutes, stirring constantly. Add wine and cook 3 minutes until mostly absorbed. Add broth and tomatoes; bring to a boil over high heat. Cover and reduce heat to medium-low or low to maintain an active simmer.
Simmer covered 30 minutes, stirring every 10 minutes. Remove lid and continue to simmer 5 to 10 minutes until most of liquid is absorbed and farro is slightly chewy but done.
Remove from heat and stir in peas, cheese, lemon juice and zest. Stir in spinach; let stand 5 minutes before serving with additional Parmesan, if desired.
Electric Pressure Cooker
Heat oil on Sauté in a 6-qt. electric pressure cooker. Add onion and mushrooms, salt and pepper; cook 8 to 10 minutes, stirring frequently, until onions are softened and most of liquid from mushrooms has evaporated.
Add farro, garlic and rosemary; cook 2 minutes, stirring constantly. Add wine and cook 3 minutes, until mostly absorbed. Press Cancel.
Add broth and tomatoes. Close lid and set to Pressure Cook on high 10 minutes. Let the steam release naturally 10 minutes before releasing the remaining steam. Remove the lid and stir. If a fair amount of liquid remains, cook on Sauté about 2 minutes, then press Cancel. Stir in peas, cheese, lemon juice and zest. Stir in spinach; let stand 5 minutes before serving with additional Parmesan, if desired.