Cook barley in a medium pot of boiling salted water until tender. Drain and spread out the barley on a baking sheet.
In a pan, add some olive oil, sauté onion and garlic. Toss in mushrooms and cook them until done. Season with salt and pepper and toss well.
Toss warm barley, cilantro, parsley, lemon juice, in a large bowl to combine; season with salt and pepper. Add mushrooms; toss again to combine. Serve immediately.