Butter a 10-by-14-inch baking dish. In a large skillet, melt 1 tablespoon of the butter in the olive oil. Add the mushrooms, season lightly with salt and pepper and cook over moderately high heat until the mushrooms start to exude their liquid, about 3 minutes. Reduce the heat to moderate and continue to cook, stirring occasionally, until the liquid has evaporated and the mushrooms are browned, about 20 minutes. Season with salt and pepper. Transfer half of the mushrooms to the prepared baking dish and the other half to a medium bowl. Cover the mushrooms in the bowl with plastic wrap and refrigerate.