Slice or coarsely chop 1/4 to 1/2 pound of mushrooms per diner, according to your taste for mushrooms, and half or all of a medium-sized yellow onion. Saute the mushrooms in approximately a tablespoon of oil, butter or bacon fat, until they’ve released their moisture, and it has evaporated. Add the onions, and cook gently until they’re lightly browned and aromatic. Transfer the onion and mushroom mixture to a bowl, and set it aside.