How to Make It
Heat 2 tablespoons of the olive oil in a large skillet. Add the mushrooms and cook over high heat, stirring, until tender and lightly browned, about 5 minutes. Add the demiglace and cook over moderate heat for 3 minutes, stirring occasionally. Season with salt. Mound the mushrooms on warmed plates and top with the Parmesan shavings. In a bowl, toss the arugula with the remaining 1/2 tablespoon of olive oil and a pinch of salt. Pile the arugula on the mushrooms and serve.