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1
Preheat oven to 350 degrees F. In a 10- or 12-inch ovenproof nonstick skillet over medium heat, heat 2 tablespoons oil. Add onion and 1/2 teaspoon salt, and cook, stirring occasionally, until onion is soft and translucent, 8 to 10 minutes. Transfer onion to a large bowl and set aside.
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2
In the same skillet over medium heat, heat 2 tablespoons oil. Working in batches, add half of potatoes in an even layer and season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook, stirring occasionally, until potatoes begin to soften, 8 to 10 minutes.
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3
Add eggs and olives to reserved onion-potato mixture, and gently stir to combine.
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4
Wipe the skillet clean. Over medium heat, heat remaining olive oil. Add half of reserved potato-onion-egg mixture to skillet, smoothing with the back of a spoon to create an even layer. Top with cheese; then cover with remaining potato mixture, smoothing with the back of a spoon to create an even layer. Cook for 10 minutes on stovetop, then transfer skillet to oven.
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5
Bake until eggs are set and potatoes are tender, 25 to 30 minutes. Adjust oven to broil and cook tortilla until top is lightly browned, about 3 minutes. Transfer pan to a wire rack and let cool for 5 minutes. Invert onto a serving plate and cut into 6 or 12 wedges. Serve hot or at room temperature.
