1. Clean mushrooms and remove stems, reserve for other use. Place caps on a plate with the gills up.
2. In a small bowl, combine the oil, diced onion, minced garlic and vinegar. Pour mixture evenly over the mushroom caps and let stand for 1 hour (approx).
3. Grill over hot grill for 10 minutes. Serve immediately. - Can cook in a skillet on the Rogue.