Slice the potatoes (peeled or unpeeled) into disks between 1/4 and 1/8 inch wide. (Helps to use a mandoline for this purpose.)
Fry potato slices in batches:
Pour the olive oil into a cast-iron or hard anodized aluminum pan (a pan that will be able to take the heat of a broiler in the oven) and heat it over medium-high heat. When it is hot enough to sizzle the potatoes as soon as they hit the oil – test with an end piece – start frying them in one layer.
Working in batches, fry until they are lightly browned, not crispy. When they are done, let the potatoes dry on a paper towel and salt them well. Keep frying until they are all cooked.
Sauté onions, arrange fried potato slices on top:
When the potatoes are all done, pour off all but a tablespoon of the oil and sauté the onions. When the onions are just starting to crisp, turn off the heat.
Arrange the onions so they are evenly covering the bottom of the pan.
Arrange the potatoes on top of the onions in a scalloped pattern.
Add beaten eggs:
Return the heat to medium, and add the beaten eggs, garlic and rosemary. Add a generous pinch of salt and pepper. Shake the pan to get the eggs to coat everything in the pan.
Let this cook until you see the edges of the tortilla begin to set, then put the pan under the broiler.
Broil to brown the top:
Cook it under the broiler for 5 minutes, or until the top browns. (If you prefer to finish the tortilla on the stovetop instead of the broiler, slide the tortilla out of the pan onto a plate. Then invert the pan over the plate, and flip the whole thing, pan and plate so that the tortilla falls back into the pan, less-cooked side down.)
Remove the pan and set it to cool for at least 5 minutes. (Be careful, the handle is hot! Drape with a potholder.)
Cut into wedges and serve:
It’s also good at room temperature, and will keep a couple days in the fridge.
More options: top with sautéd onions, chorizo, sour cream, sharp cheddar and salsa!