Clean mushrooms with a damp paper towel. (Washing them under water can make them turn slimy.)
Depending on the size of the mushrooms you can slice them (about 1/2" thick with or without stems or cook the tops whole.
Heat 2tbs oil in a large skillet or frying pan on a medium/low flame.
Add mushrooms and cook for 3-4 minutes a side until they are still a little crunchy but not raw.
Add the herb and coarsely chopped garlic and cook for a minute or so.
Add 6 tbsp Balsamic vinegar, mix well and cook for 3-4 minutes to reduce the liquid to half the volume. The idea here is cook off the tanginess of the vinegar until it is sweet and slightly syrupy.
Turn fire off and finish by quickly blending in a tablespoon of cold butter.
Add salt and ground black pepper to taste, garnish with parsley and serve.