In a large heavy skillet heat the oil over moderate heat. Add the potato slices, one at a time to prevent sticking, in a single layer. Alternate potato layers with the onion slices and season with salt and pepper. Cook the vegetables, lifting and turning the potatoes occasionally, until tender but not brown. In a large bowl beat the eggs with a fork. Transfer the tender potatoes and onions to a colander to drain. Reserve 3 tablespoons of the olive oil. Add the drained potatoes and onions to the eggs, pressing down on the potatoes so that they are completely submerged, and let sit for 15 minutes.
Heat 2 tablespoons of the reserved oil in a large heavy skillet over high heat. Add the potato-and-egg mixture, quickly spreading it out in the pan with a spatula. Sprinkle half of the herbs over the egg mixture. Lower the heat to moderately high and shake the pan occasionally to prevent sticking. When the potatoes begin to brown underneath, invert a large plate over the skillet. Flip the omelet onto the plate. Remove any pieces that have stuck to the pan and arrange them on the tortilla. Add the remaining tablespoon of reserved oil to the pan, then slide the tortilla back into the skillet. Lower the heat to medium and continue to cook the tortilla, flipping 2 to 3 more times, until it is firm but still moist. Transfer the tortilla to a platter and sprinkle with remaining herbs. The tortilla can be served hot, warm, room temperature, cool, or cold cut into wedges.
Recipe courtesy of Gourmet Magazine