Heat a large skillet over high heat. Add the mushrooms in a single layer, being careful not to overcrowd, and cook, working in batches if necessary, until mushrooms begin to just brown, about 7 minutes.
Reduce the heat to medium-high. Sprinkle the mushrooms with the salt and cook, undisturbed, until mushrooms start releasing their moisture, about 5 minutes. Add the olive oil and cook, stirring occasionally, until mushrooms are tender and edges are golden brown, about 8 minutes more.
Add the butter to the pan. Once the butter melts, add the garlic and cook, stirring often, until garlic is fragrant and mushrooms are coated, 2-3 minutes.
Sprinkle mushrooms with parsley and serve.
Serves 4