In a large saucepan, heat the olive oil. Add the mushrooms and garlic and season with salt and pepper. Cook over moderately high heat, stirring, until the mushrooms are softened and the garlic is fragrant, about 4 minutes. Add the barley and stir to coat. Add 1 cup of the warm stock and cook over moderately low heat, stirring constantly, until nearly absorbed. Continue adding the stock 1 cup at a time, stirring constantly between additions, until the barley is al dente and suspended in a creamy sauce, about 25 minutes. Stir in the butter and cheese. Season with salt and pepper and serve.