You will thank me for this. Your scale and waist-line will not, however, so please just don’t tell them where you learned it.
After you’ve sauteed something in a pan, the brown (yummy) bits on the bottom of the pan are called “fond”. You should be fond of your fond because it will do heavenly things for you if you treat it fondly. (okay…that’s enough of that…)
caramelize some chopped onions, add butter (mash up some flour in your butter if you want a thicker “gravy” type sauce), stock, white wine, cream (the good stuff! don’t scrimp!), and mushrooms. Whisk it all together, making sure to pick up the fond from the bottom and sides of your pan. Low-ish heat will do. How much of each ingredient varies widely on your tastes and what kind of sauce you’re going for. I suggest 1/4c wine, 1c stock, and 1/2c cream…but really, I always just eyeball it.