Use a wet paper towel or quick dunk (described above) to clean the mushrooms. Cut off the stems. Save for stock, they provide a nice flavor.
Cut caps into 1/2″ slices.
Heat a 12″ heavy skillet on medium high heat until hot.
Add oil or butter to pan. When shimmering add mushrooms to the pan and stir to coat as evenly as possible in the oil. DO NOT add salt to the mushrooms at this point.
Cook, stirring occasionally, for 10-15 minutes, until nicely browned in spots and tender.
Stir in the balsamic and allow the mushrooms to absorb the liquid. Turn off the heat.
Season with salt and pepper to taste.
To serve, toss with a little more olive oil, or butter and garnish with parsley if you like.