Tortilla Española (its relation to Mexican tortillas comes solely from it round shape) is among the most popular dishes in Spain. Although its ingredients couldn't be more basic--potatoes, eggs, onions, and oil--they're combined and cooked in a way that makes this dish irresistible and versatile. The potatoes are normally fried, but we've roasted them with excellent results. Unlike American omelets, this one's best made several hours ahead then served at room temperature.
Place the potato and onion in a roasting pan coated with cooking spray. Drizzle with 1 tablespoon plus 2 teaspoons oil, and sprinkle with 1/2 teaspoon salt. Toss well. Bake at 350° for 1 hour or until potatoes are tender, stirring occasionally with a metal spatula to prevent sticking.
Combine eggs and 1/4 teaspoon salt in a large bowl. Stir in potato mixture; let stand 10 minutes. Heat 1 teaspoon oil in an 8-inch nonstick skillet over medium heat. Pour potato mixture into pan (pan will be very full). Cook 7 minutes or until almost set, gently shaking pan frequently.
Place a plate upside down on top of omelet; invert onto plate. Carefully slide omelet cooked side up into pan; cook 3 minutes or until set, gently shaking pan occasionally. Carefully loosen omelet with a spatula; gently slide omelet onto a plate. Cool. Cut into wedges. Garnish with oregano, if desired.