Melt the salted butter (2 tablespoons) in a large, preferably nonstick skillet over medium-high heat.
Add the sliced mushrooms and the kosher salt (1/4 teaspoon) to the skillet. Stir to coat in the butter, and spread the mushrooms out in an even layer, with as much surface area in contact with the skillet as possible.
Allow the mushrooms to cook, WITHOUT STIRRING, for about 5 minutes, until much of the liquid has been drawn out of the mushrooms and has almost fully evaporated.
Stir the mushrooms, trying to flip them over as much as possible (but not being obsessive about it). Spread them in an even layer again, and allow to cook for another 2-3 minutes without stirring.
Stir one more time and allow to cook for another 2-3 minutes, until mushrooms are deeply browned and there is almost no liquid in the pan.
Adjust seasoning if needed, adding black pepper if desired. Serve as is, or use in other recipes such as pasta or quiche.
If using unsalted butter, increase the amount of kosher salt to 1/2 teaspoon.
For a vegan version, use olive oil instead of butter.
For a paleo/whole30/dairy free version, use ghee or olive oil instead of butter.
You can wash the sliced mushrooms in a colander under running water- it's not necessary to wipe them clean with a towel to keep them dry. Just shake off as much excess water as you can.
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.