In a large mixing bowl, combine olive oil, salt/pepper to taste, garlic, balsamic vinegar, onion powder and thyme leaves.
Clean mushrooms and remove stems. Wipe them with a damp cloth.
Marinate the mushroom in an olive oil mixture for at least 15 minutes.
Preheat the oven to 400°.
Place the mushrooms on a baking sheet.
Roast for about 10-15 minutes or until tender.