Peel the potatoes and slice into ⅛-inch thick rounds. Slice the onion into thin half-moons. Pat the potato and onion slices dry thoroughly with paper towels to remove excess moisture.
2 lbs potatoes, 1 medium onion
Heat the olive oil in an 8 to 10 inch skillet over medium-low heat. Add the potatoes, onion, and half the salt (½ tsp). Cook gently for about 20 minutes, stirring occasionally, until the potatoes are fork-tender but not browned. Use a slotted spoon to transfer the potatoes and onions to a plate lined with paper towels and drain for 5 minutes. Reserve 2 tablespoons of the oil and discard the rest.
1 cup olive oil, 1 tsp salt, 2 lbs potatoes, 1 medium onion
Crack the eggs into a large bowl. Add the remaining salt (½ tsp) and black pepper. Whisk vigorously for 1 minute until the mixture is completely smooth and homogenous. Gently fold the drained potatoes and onions into the beaten eggs, being careful to keep the potato slices intact.
6 large eggs, 1 tsp salt, 0.5 tsp black pepper, 2 lbs potatoes, 1 medium onion
Heat the reserved 2 tablespoons of olive oil in the same skillet over medium-low heat. Pour in the egg and potato mixture, spreading it evenly. Cook without stirring for 5 to 6 minutes until the edges are set and the bottom has a golden crust. The center should look slightly wet but not runny.
6 large eggs, 1 cup olive oil
Place a large plate over the skillet and quickly flip the tortilla so it lands on the plate. Slide the tortilla back into the skillet, uncooked side down. Cook an additional 4 to 5 minutes until the center feels just firm when pressed gently. Slide the tortilla onto a serving plate and let it rest for 5 minutes before slicing and serving.
6 large eggs
