Using a meat mallet, moisten the steaks with a little water and place between two pieces of plastic wrap and pound out to a thickness of about 1/4 inch.

Heat the olive oil in a frying pan at medium heat. Add the onions and sauté briefly. They should still be slightly crisp. Remove onions. (If you are using mushrooms, add them and remove at the same time as the onions.)

Sprinkle each steak with salt, black pepper, cumin, and paprika and rub it in.

Add the garlic to the pan and sauté for 1 minute only. Immediately add the steaks and sauté to desired doneness, turning once only. These thin steaks will cook very quickly. Medium-rare calls for a very quick flip -- no more than a minute or so on each side.

Remove the steaks and place them somewhere where they'll stay warm and away from the grasping mitts of your hungry sister-in-law. Deglaze the pan by adding the lime juice and the butter and continuing to heat, stirring constantly. Cook long enough to reduce (thicken) somewhat.

Cover each steak with the sautéed onions and mushrooms. Pour the sauce over the steaks and sprinkle with chopped cilantro.