How to Make It
Heat the oil in a large skillet over medium-high heat. Add the mushrooms and ½ teaspoon salt and cook, stirring occasionally, until the liquid is released and they begin to brown, 7 to 9 minutes.
Combine the mushrooms, wine, vinegar, onion, oregano, garlic, 1 teaspoon salt, and ¼ teaspoon pepper in a medium bowl. Cover and refrigerate at least 8 hours and up to 5 days.