Heat the oil and butter in a 12-inch sauté pan or skillet over medium heat until the butter foams. Add the mushrooms and garlic. Like sponges, the mushrooms will immediately absorb all the fat in the pan. Sprinkle with the salt and stir with a wooden spoon until the mushrooms start to release their moisture and begin to shrink, 2 to 3 minutes. Increase the heat to medium high so that you hear a steady sizzle; stir occasionally. In about 5 minutes, when the liquid evaporates and the mushrooms start to brown, give just an occasional sweep with the spoon (about once a minute) to allow the mushrooms to brown nicely, cooking them another 2 to 4 minutes. Resist the inclination to stir too often. Turn off the heat and toss the mushrooms with the parsley and pepper to taste, adding more salt if needed. If serving as a side dish, stir in a few tablespoons cream, broth, or lemon juice to moisten the mushrooms and to deglaze the pan, scraping the browned bits off the bottom of the pan into the mushroom mixture. Add other herbs if you like.