Prepare the Swiss chard mixture: Heat oil in large skillet over medium heat. Add onions, cook and stir until translucent, about 5 minutes. Increase heat to medium high and add chopped Swiss chard, all of it, and press down and stir until wilted. Add salt, pepper, sage and nutmeg and continue cooking and stirring until no liquid is left in pan, 5 to 10 minutes. Preheat oven to 400º.