The potatoes and onions
Virgin olive oil—not extra virgin—is the classic choice for tortilla making. Alternatively, you can blend extra virgin olive oil with a neutral high-heat oil (like grapeseed).
The old-school method is to use enough oil to allow the potato slices to separate and swim around a bit—maybe about 1 1/2 cups of oil. With a non-stick pan, Raquel, José, and Tricia get away with less than 1/4 cup of oil.
Cook the potatoes and onions
Salt the potatoes and onions lightly, add them to the oil, and cook for about 10 minutes over medium-high heat, stirring frequently, until the onions are softened and the potatoes are cooked through—a slice of potato should be easily split with a spoon.
Drain the cooked potatoes and onions in a colander, reserving the oil for the next phase of cooking.
Many Spanish home cooks will let the potatoes and eggs rest for up to 15 minutes before proceeding. It works out great if you let it sit for as long as it takes you to get your tortilla skillet ready.
Start the tortilla in a very hot skillet
Reduce the heat to medium-low. Run a heat-proof spatula around edges to make sure the tortilla doesn’t stick.
Cook the second side
The total cooking time will vary depending on the thickness of your tortilla. My tortillas generally take about 15 minutes for the eggs to set through the middle.