Bring a medium saucepan of heavily salted water to a boil over high heat. Stir in the barley, reduce the heat to medium, and cook until al dente, about 15 to 20 minutes. Drain and set aside.
Heat 2 tablespoons of the butter in a large frying pan over medium heat. When the foaming subsides, add the onion, season with salt and pepper, and cook until translucent, about 3 minutes. Add the mushrooms, increase the heat to medium high, and cook, stirring rarely, until golden brown, about 10 minutes. Add the wine and scrape up any browned bits from the bottom of the pan.
Add the green beans and broth, season with salt and pepper, and cook until the beans are fork tender and the broth is slightly reduced, about 5 minutes. Stir in the reserved barley and cook until heated through, about 1 minute. Stir in the parsley and remaining 1 tablespoon butter and season with salt and pepper.