Preheat oven to 425 degrees and line a baking sheet with aluminum foil or spray a baking dish with non-stick cooking spray.

Wipe mushrooms with a damp paper towel and trim any long stems. Cut large mushrooms in half or quarter them if they're huge.

In a mixing bowl, add oil, vinegar, garlic, thyme, salt, and pepper, and whisk until well combined. Toss the mushrooms in the mixture to coat them.

Spread the mushrooms out on the baking sheet and scatter butter pieces throughout.

Bake for 20-25 minutes, shaking the pan or stirring the mushrooms halfway through.

If desired, sprinkle with fresh minced parsley before serving.