Unlike arborio rice in risotto, farro doesnt become gummy; instead, it retains its tender, distinct bite, even if it sits a while after cooking. Serve with a tossed green salad.
In medium saucepan, bring stock to a boil. Reduce heat to low.
In large saucepan, heat oil and 1 tablespoon butter over medium-high heat. Add both mushrooms and cook, stirring often, until soft, about 5 minutes. Using slotted spoon, transfer mushroom mixture to bowl and set aside. Add remaining tablespoon of butter and onion and cook, stirring often, until soft, about 5 minutes.
Add farro to pan and stir to coat with oil. Add wine and cook, stirring until liquid is absorbed, about 1 minute. Add 1 cup hot stock to pan and stir in mushrooms. Cook, stirring occasionally, adding more hot stock as it is absorbed, until farro is tender but still firm, about 20 minutes.
Meanwhile, make tomato sauce. In small saucepan, heat oil over medium-high heat. Add onion, garlic and bell pepper and cook, stirring often, until vegetables are soft, about 5 minutes. Add tomatoes, sugar, salt and pepper. Cook, stirring occasionally, until thickened slightly, 15 to 20 minutes.
To serve, stir grated cheese, if using, into farro and season with salt and pepper. Spoon into serving bowls and sprinkle with chopped parsley if desired. Top each serving with tomato sauce.