With a damp paper towel, wipe off the tops of the mushrooms to remove as much dirt as possible.
Cut the mushrooms in half through the stem. If you have smaller mushrooms, you can leave them whole.
In a medium sauté pan, over medium heat, melt the butter and olive oil.
When the butter begins to foam a bit, add the mushrooms and salt. That salt will help release the juices in the mushrooms which is exactly what you want to happen. Stir just a bit to incorporate the ingredients together, and then, cover and sauté until the mushrooms release a good bit of their juices and start to turn a golden brown, stirring occasionally. This should take about 10 minutes.
Add the garlic, parsley and pepper. Stir to incorporate and continue to sauté, uncovered, for 1-2 minutes. The mushrooms will start to shine and render more of their juices.
Stir in the crème fraîche, turn the heat up to medium high, and cook until the sauce starts to thicken a bit, about 1-2 minutes. It will happen quickly, so don't walk away....or check your phone!
Transfer mushrooms and their sauce to a shallow bowl and serve immediately.