This side dish tastes just as good at room temperature as it does hot from the stove, so there's no need to worry if it's ready before the rest of the meal.
Heat oil in a skillet over medium-high heat. Add mushrooms, and cook until golden brown, about 5 minutes. Stir in vinegar, salt, and red-pepper flakes, and season with pepper. Cook 1 minute more. Transfer to a bowl, and serve.