Wipe mushrooms with a slightly damp paper towel and then set aside.
Melt butter in large skillet over medium-high heat. Add mushrooms, onion and garlic; cook for about 8-10 minutes or until mushrooms are tender, stirring frequently.
Add sea salt, pepper and balsamic vinegar to the pan and stir to coat. Cook for another 2 minutes. Sprinkle with fresh chopped parsley, if using.