Wash and dry the chard leaves well (I often let them soak for about 30 mins in salted water to remove some of the bitterness). Slice them very thinly and place in a large bowl with the chopped parsley leaves. In a small bowl, whisk the eggs and add to the chard, adding three eggs at first and a fourth (or even a fifth) if necessary. Grate the parmesan and stir it into the mixture along with the seasonings.
Warm the oil over medium heat in a heavy frying pan (24 cm seems to be an ideal size) with a tight-fitting lid. Add the chard and press down with a wooden spoon to flatten the mixture. Cover, lower the heat a little and cook for about 15 mins, until the base is browned. Keep an eye on it to be sure that it doesn't cook too quickly or slowly. Now place a large plate over the pan, put on some oven mitts and flip the omelette over onto the plate. Slide it back into the frying pan, cover again and cook for another 5-10 mins, until lightly browned on the other side. Serve hot, warm or cold.