Serves 8-10 for tapas or 4-6 for dinner Difficulty
3/4 cup olive oil 1 medium onion, finely chopped 4 medium-large russet potatoes, peeled and cut into 1/8" slices Salt 5 large eggs
Heat a large skillet and add oil. When oil is hot, add onions and cook over low heat for about 20 minutes. Onions should not burn. Discard onions and add a layer of potato slices. Salt the layer and repeat with all of the potatoes, salting each layer. Cook slowly, simmering in oil, until all potatoes are tender. The potatoes should not brown. Lift and turn potatoes carefully as they cook. Remove cooked potatoes from pan and drain.
Reserve about 2 Tablespoons of oil and wipe out skillet with a paper towel.
In a large bowl, beat eggs with a fork and add a pinch or two of salt. Add potatoes and press down to immerse into eggs. Let mixture sit for 10 minutes. Return reserved oil to skillet and heat to the smoking point. Add potato egg mixture all at once. Press down to spread mixture evenly in pan. Lower heat to medium and cook until the tortilla is lightly browned on the bottom. Place a plate on top of skillet upside down and turn tortilla over onto plate.
Add about a teaspoon of oil to skillet and let it get hot again. Slide tortilla back into skillet, cooking until lightly browned on that side. Turn tortilla two more times, about 30 seconds on each side. Serve tortilla by cutting into wedges. Serve warm, or at room temperature.