Make sure your greens are very dry before preparing recipe. Also, wash and chop them when you come in from market, then they are ready for you to cook up even quicker.
Heat the oil in a large nonstick skillet over medium-high heat. Add onions and toss 2 minutes, then add chard in bunches and keep it moving as it wilts up a bit - you are just searing it up. The greens should remain crisp and crunchy. Wilting them all and searing them up should take no more than 3 to 4 minutes. Season the greens with nutmeg and salt and pepper, to taste. Douse the pan with a little vinegar and remove from heat. Toss to cook off vinegar and serve the greens hot.