In a large nonstick skillet set over moderate heat, warm 1 tablespoon of the oil until it is hot, add the onion, rosemary, thyme, salt and pepper and cook, stirring occasionally, for 5 to 7 minutes, or until onion is golden. Let cool.
In a bowl whisk together the whole eggs, egg whites, and some salt and pepper to taste. Add the onions and stir to combine.
In the skillet heat 1 tablespoon oil until hot over moderate heat, add the egg mixture and cook just until edges begin to set, about 3 minutes. Add the potatoes, spreading them in an even layer, reduce the heat to moderately low, and cook, covered, for 6 to 8 minutes, or until eggs are almost completely set. Place a plate over the omelette and invert the omelette onto the plate. Set the skillet back on the heat, add remaining oil, turning pan to coat bottom with oil, and slip omelette back into pan. Cook for 3 minutes more, or until bottom is just set.
Transfer omelette to serving dish. Cut into wedges and serve hot or warm.