Add 3 tablespoons of the reserved oil back to the skillet. Heat the oil in the skillet over moderately high heat until very hot but not smoking (it must be hot or the eggs will stick): add the potato and egg mixture, spreading the potatoes evenly around the pan. Reduce the heat to low, and shake the pan often and run a spatula around the side and bottom to make sure the omelet is not sticking. After approximately 8 to 10 minutes, when the omelet is cooked 3/4 of the way through (top is no longer liquid) and the bottom is beginning to brown, place a large plate over the pan and invert the omelet onto the plate