In a medium saucepan of boiling water, cook the barley until tender, about 30 minutes. Drain and transfer to a large bowl.

In a large skillet, heat 2 tablespoons of the olive oil. Add one-third of the mushrooms and cook over high heat, stirring, until golden brown, about 5 minutes. Transfer to the large bowl. Repeat in 2 more batches with 4 tablespoons of the oil and the remaining mushrooms. Add the sage, lemon juice and the remaining 1/4 cup of oil to the bowl; season with salt and pepper and toss to coat.