Heat 4 tablespoons olive oil in a 12- to 14-inch nonstick skillet over high heat. Add potatoes, season highly with salt and pepper and saute, tossing often, until tender, about 10 minutes. Drain potatoes in a sieve set over a bowl. Let cool. (If you have only a smaller non-stick skillet, fry the potatoes in two batches, using 2 tablespoons oil each time.)

Return skillet to moderate heat and add 1 tbsp olive oil. When hot, add leeks, season with salt and pepper and toss to coat with seasonings. Cover, reduce heat to moderately low and cook until leeks are tender, about 15 minutes. Let cool.

In a large bowl, combine eggs, potatoes, leeks, peas and fava beans. Season with salt and pepper, stir well, then let stand for 10 minutes.

Measure oil drained from potatoes, if any. Add enough olive oil to make 1 tablespoon. Heat a 10-inch skillet over high heat. When hot, add oil. When oil is hot, add egg mixture, spreading it into an even layer. It will bubble vigorously around the edges. Immediately lower heat to moderately low and cook 10 minutes. The omelet will still be underdone on the surface. Invert a cookie sheet over the skillet. Grasping the skillet handle with one hand and holding the cookie sheet in place with the other, flip the skillet so the tortilia falls onto the cookie sheet. Immediately slide it back into the skillet, cooked side up. (Some moist bits of egg may stick to the cookie sheet; that's okay.) Cook an additional 10 minutes, then invert onto a serving platter. Let stand at least 20 minutes before serving.