That's actually pretty easy. Get sliced mushrooms (or slice 'em yourself if you like) and put them in a skillet or sautee pan with a bit of butter (about 1/2 Tbl per 4 oz.). Cook them on medium high heat, stirring fairly often, until you've cooked the water out and they start getting dark. Deglaze the pan with a splash of white wine, and let the liquid cook off, as you get the good bits stuck on the bottom of the pan back onto the mushrooms. Serve immediately.
Same technique works for grilled onions.
Oh, I forgot that you should hit the mushrooms/onions with salt and pepper at the start of the cooking, too.