Wipe the mushrooms gently, using paper towels (don't just rinse them, because they'll suck up a lot of water). Heat the oil in a frying pan and add everything but the salt, stirring to make sure everything gets well coated in the oil.
Fry for about 7 or 8 minutes, stirring occasionally, until the mushrooms and onions are tender. Season to taste with the salt and you're ready to serve! Note: Fresh cilantro makes a great garnish for this dish, as it could use a little color.