TO ROAST THE MUSHROOM CAPS:
Preheat the oven to 400°F. Remove the stems from the mushrooms and wipe the caps clean, lightly oil a baking pan with some of the olive oil. Rub the remaining oil into the tops of the caps. Sprinkle the mushrooms with salt and pepper.
Place the caps, gill side up, on the baking pan. Roast for 10 minutes. Turn and roast until the mushrooms are tender and well browned, about 15 minutes.
Remove the pan from the oven and sprinkle the mushrooms with vinegar, if desired.
Let cool for 2 to 3 minutes before serving.
TO GRILL THE MUSHROOM CAPS:
Preheat a grill to hot. Rub the oil into all sides of the mushroom caps.(You may need a little more than 2 tablespoons oil for this.) Sprinkle the mushrooms with salt and pepper.
Place the mushrooms, gill side up, near the coolest part of the fire. Grill the caps until well marked and brown, about 8 minutes. Turn the caps and continue grilling until the mushrooms are tender, about 10 minutes. Move the caps occasionally during grilling to prevent overbrowning.
Remove the caps to a platter and sprinkle with vinegar, if desired.