Bring a medium pot of salted water to a boil. Add the farro and cook, stirring occasionally, until al dente, about 25 minutes; drain.
Meanwhile, heat the oil in a large skillet over medium-high heat. Reduce the heat to medium, add the onion and a pinch of salt and cook, stirring often, until soft and pale golden, about 15 minutes. Increase the heat to medium-high and add the butter. When melted, add the turnip, greens and apple and cook, stirring, until everything is tender and golden brown in spots, 10 to 12 minutes more. Add the farro and crushed red pepper to the skillet and stir to combine. Season with salt and serve warm.