Slice mushrooms into thin segments and chop onion fine. Heat a skillet to medium and add the ghee (or olive oil, coconut oil, or vegan spread of your choice). Sauté the onions first, for about two minutes, to just soften. Then add mushrooms and reduce heat to low. Sauté until mushrooms are lightly but not completely cooked, about 3 minutes; remove from heat before they spill a lot of water into the skillet. If they do, drain it off before placing in your pie crust later.