Step 1

Place potatoes in a large colander, sprinkle with 2 tsp. salt, and toss to coat. Let stand for 30 minutes. Pat dry with paper towels.

Step 2

Heat oil in a large nonstick skillet over medium-high heat. Add potatoes and cook, turning occasionally, until crisp-tender, 10–15 minutes.

Step 3

Add onion to potatoes in skillet; cook until onion and potatoes are soft, 5–8 minutes. Using a slotted spoon, transfer mixture to a large bowl; let cool slightly. Strain oil into a heatproof bowl; set aside.

Step 4

Add eggs to potato mixture; season with salt. Wipe out skillet. Heat ¼ cup reserved oil over medium-high heat (chill remaining oil for another use). Add egg mixture, arranging potatoes to submerge.

Step 5

Reduce heat to medium; cook tortilla until almost set (it will be slightly runny on top), 25–30 minutes. Invert onto a large plate. Slide into skillet browned side up. Cook until golden on bottom and cooked through, about 4 minutes more. Serve warm or at room temperature. Cut into wedges.

Step 6

DO AHEAD: Tortilla can be made 2 hours ahead. Let stand at room temperature.