Giant Mushroom Burger

Heat oven to 425-F. Place racks in the top third and middle of the oven. Wipe mushroom caps clean with damp paper towels. In an small bowl, stir together oil, vinegar, garlic and herbs. Add salt and freshly ground pepper to taste. Bruch mushroom caps with oil mixture, working herbs and garlic into the gills on the underside of the mushrooms. Brush onion slices lightly with oil. Place mushroom caps and onion slices on a lightly greased baking sheet. Place mushrooms and onions on top rack of oven. Bake mushrooms 20 to 25 minutes, or until centers are just cooked through. Remove from oven and place cheese slices on top of mushroom caps. Return baking sheet to oven for 1 or 2 minutes, or until cheese melts. Place mushroom caps on rolls with onion slices. Add tomato and arugula and condiments such as mustard, mayonnaise or ketchup, as desired.