1. Combine the dried oregano or marjoram, olive oil, balsamic vinegar, and thyme in a medium bowl.
2. Add the mushrooms into the bowl and mix well to combine. Set aside to marinate for a minimum of 30 minutes at room temperature or overnight, refrigerated.
3. Once the mushrooms are marinated, spread them in single layer on a hot grill and cook until tender (about 6 minutes); or spread the mushrooms on a baking sheet and roast in the oven at 450 degrees F. Mushrooms may also be cooked in olive oil in a large sauté pan over high heat until tender (4 to 6 minutes).