Heat 2 Tablespoons oil in a 12-inch nonstick skillet over medium high heat until just smoking. Add 2 pounds mushrooms and 1 teaspoon salt and cook, covered and stirring occasionally, until the mushrooms release their liquid and begin to soften, about 10 minutes. Uncover and continue to cook, stirring occasionally, until skillet is dry and mushrooms are lightly browned, about 5 minutes.
To the pan, add 1-2 sliced shallot, 3 minced garlic cloves, 1 teaspoon oregano, and ½ teaspoon red pepper flakes. Cook until fragrant, about 1 minute. Stir in ¾ cup vinegar and ½ cup water and bring to a boil. Cook until liquid reduces by half, about 5 minutes.
In a medium bowl, whisk together the remaining 6 Tablespoons oil, 1 generous Tablespoon Dijon mustard, ¼ teaspoon salt, and ¼ teaspoon pepper.
Add the mushroom mixture to this bowl and stir to combine. Let cool completely. Stir in parsley.
Cover mushrooms and refrigerate until chilled, about 1 hour. Season with salt and pepper to taste before serving.
Mushrooms can be refrigerated up to 3 days.