Preheat the oven to 350°. In a deep skillet, heat the olive oil. Add the leek and cook over moderate heat, stirring, until softened, 3 minutes. Stir in the butter and mushrooms. Cover and cook, stirring occasionally, until the mushrooms are softened and a lot of liquid is released, 7 minutes. Uncover and add the soy sauce and red wine and cook over moderately high heat, stirring, until the liquid is reduced to 2 tablespoons, 5 minutes. Add the spinach and stir until wilted, 2 minutes. Season with salt and pepper.