Trim the tough stems from the spinach. Coarsely chop the leaves and wash carefully in 2 or 3 changes of water to remove all sand. Drain.
Heat the oil or butter in a very large sauté pan or skillet over medium heat. Add the onion and cook 5 minutes. Add the garlic and the spinach leaves and cook in their own moisture, stirring often, until the spinach wilts, about 4 m