In Spain, tortilla refers to a savory round based on potatoes or eggs, not the Mexican flatbread of flour or corn.
3/4 cup olive oil
4 medium potatoes, pared and cut into 1/8-inch slices
1 medium onion, cut into 1/8-inch slices
1/2 teaspoon salt
1/4 teaspoon pepper
Heat oil in 10-inch nonstick skillet until hot. Layer potato and onion slices alternately in skillet. Cook over medium-low heat, turning frequently, until potatoes are tender but not brown, about 12 minutes. Remove potatoes and onion with slotted spoon; drain, reserving 3 tablespoons oil in skillet.
Beat eggs, 1/4 teaspoon of the salt and the pepper in large bowl; gently stir in potatoes and onion. Sprinkle remaining salt over potatoes.
Heat oil in skillet until hot; pour egg and potato mixture into skillet. Cook uncovered over medium-low heat until potatoes begin to brown on bottom and edge of omelet is firm (center will be wet), about 7 minutes.
Shake pan occasionally to prevent omelet from sticking. Place large plate over skillet; invert omelet on plate. Slide omelet back into skillet. Continue to cook over medium-low heat until eggs are set and potatoes are golden brown, about 2 minutes longer. Turn onto serving plate; cut into wedges to serve.
Yields 4 servings.