Preheat oven to 425 degrees, and line a baking sheet (with a rim) with aluminum foil. Alternatively, spray a baking dish with non-stick cooking spray.

Wipe mushrooms with a damp paper towel and trim any long stems. If mushrooms are large, cut in half, or quarter if they're huge.

Add oil, vinegar, garlic, thyme, salt and pepper to a mixing bowl and whisk until well combined. Add mushrooms and toss to ensure mushrooms are coated.

Spread out on baking sheet (or baking dish) and scatter butter throughout mushrooms.

Bake for 20-25 minutes, giving the pan a shake (or stirring the mushrooms) about halfway through.

If desired, sprinkle with fresh minced parsley before serving.