Heat oil in a well-seasoned cast iron (or nonstick) skillet over medium-high heat. Add chard and onion and cook until just softened, about 2 minutes. Meanwhile, whisk eggs vigorously adding salt and chile flakes, if using. Pour eggs into pan and stir to incorporate greens and onions. Cook, stirring occasionally, until large curds form. When eggs are about 80% cooked, turn off heat and add the splash of milk, stirring to coat eggs.