I love this recipe because it is fast and delicious.  I make the mushroom mixture ahead of time and keep it in the freezer.

Slice 12-16 oz. cremini mushrooms fairly thin.  In a large saute pan, heat 1 T. olive oil (or less if you use less mushrooms as you can easily make more or less of this) over medium high heat until quite hot.  Add the mushrooms and distribute around the pan.  Leave alone for a few minutes, just giving the pan a shake every now and then.  After about 5 minutes or so, when the mushrooms have started to release their liquid season with salt and pepper.  Reduce heat to low and cook slowly for 10-15 minutes until mushrooms look dark and have lost all raw color.  If you want to make this really good at this point, turn heat to high and add either 1/3 c. Marsala or Madeira to the pan.  Boil until it has pretty much evaporated.  At this point you can let cool and put into freezer or fridge for another day.  If not, add 2 bags of baby spinach to the pan while on high heat and quickly stir fry until it just wilts (not all the spinach has to wilt).  You want it to stay a nice glossy green so it is better if it is undercooked as opposed to overcooked as the heat from the pan while continue to wilt the spinach.  This is also very good reheated.  Sometimes I refresh it by heating the leftovers over very high heat and then adding some additional spinach.