Preheat oven to 450°F.
Arrange mushrooms in a single layer on a large jelly roll pan and coat with cooking spray; season to taste with salt and pepper. Roast mushrooms for 15 minutes and then turn mushrooms and roast for 5 to 10 minutes more, until cooked through.
Meanwhile, place garlic and thyme in a large bowl. When mushrooms are done cooking, immediately place in bowl and toss to coat with garlic and thyme. Add vinegar and toss again. Yields about 1 cup per serving.