from mom, à la julia child
Heat oil and butter on medium high until the foam subsides from the butter.
CAREFULLY add sliced mushrooms -- half a pound for this amount of fat and a large skillet
Stir and sauté while they absorb the fat, begin to give it back and brown
Add flavors -- salt, pepper, maybe a few chopped green onions, or -- my style -- salt, pepper, tarragon, then a splash of sherry or other wine (2 - 3 T), which you then boil down over high heat till just the essence remains
works also well with portobello mushroom caps, but they take longer to sauté than little ones do.