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1
You will need a heavy skillet with a diameter or 9 to 10 inches (Dorie uses a cast-iron skillet; smaller is better than larger).
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2
Choose a skillet with a handle that can go under the broiler (if you are not sure, wrap the handle in foil).
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3
Pour about 2 tablespoons oil into the skillet and warm it over medium heat.
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4
Add the potatoes and onion, garlic and rosemary if using; turn the ingredients around until they glisten with oil.
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5
Season with salt and pepper; decrease the heat and cook slowly until the potatoes are golden and cooked through, about 20 minutes; you should be able to pierce the potatoes easily with the tip of a knife.
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6
Discard the garlic and rosemary and transfer the potatoes and onion to a bowl.
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7
Carefully wipe the skillet with a paper towel (if anything has stuck to the bottom of the pan, you should wash it--you need a clean surface so the tortilla will unmold easily).
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8
Position broiler rack so that when you slide the skillet onto it, the pan will be about 6 inches from the heat source; preheat broiler.
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9
In a big bowl, beat the eggs with salt and pepper to taste and the cayenne, then stir in the potatoes and onion.
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10
Put the skillet over med-high heat and pour in about 1 1/2 tablespoons oil.
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11
When the oil is hot, add the eggs and potatoes to the pan.
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12
Immediately lower the heat, and let the eggs cook, undisturbed, for about 2 minutes.
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13
Run a silicone spatula or a table knife around the edges of the pan to release the tortilla, then cover the pan and cook slowly for another 8-10 minutes, or until the top is almost set--there will be a circle of liquid or jiggly egg at the center.
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14
Every couple of minutes, run your spatula around the sides of the pan and just under the tortilla to keep it from sticking.
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15
Slide the pan under the broiler and cook until the top of the tortilla is set; check it after 1 minute, and then keep checking--it can go fast.
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16
Transfer the tortilla to a serving platter and allow it to cool to room temperature before cutting and serving it.
