Heat 2 tablespoons of the oil in a 10inch skillet. Add the onion and cook over low heat, stirring occasionally, until completely soft but not browned, about 15 minutes. Add the chard and continue cooking, stirring occasionally, until the moisture has cooked away and the chard is tender, about 15 minutes. Season well with salt and freshly ground pepper. Meanwhile, mash the garlic in a mortar with a few pinches of salt, then stir it into the eggs along with herbs. Combine the chardonion mixture with the eggs, stir in the Gruy re cheese and half the Parmesan.
Preheat the broiler. Heat the remaining oil to the skillet and when it's hot, add the eggs. Slide the pan back and forth a few times to make sure the eggs are not sticking. Keep the heat at mediumhigh for about one minute, then turn it to low. Cook until the eggs are set but still a quite moist on top. Add the remaining cheese, slide the pan under the broiler and broil until browned.
Serve the trouchia in the pan or slide it onto a serving dish and cut into wedges. The gratined top and the golden bottom are equally presentable.