Place potatoes in a large colander, sprinklewith 2 teaspoons salt, and toss to coat.
Let standfor 30 minutes. Pat dry with paper towels.
Heat oil in a large nonstick skillet overmedium-high heat.
Add potatoes and cook,turning occasionally, until crisp-tender,10-15 minutes.
Add onion to potatoes in skillet; cookuntil 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.
Add eggs to potato mixture; season withsalt. Wipe out skillet.
Heat 1/4 cup reservedoil over medium-high heat (chill remainingoil for another use).
Add egg mixture,arranging potatoes to submerge.
Reduce heat to medium; cook tortillauntil almost set (it will be slightly runnyon top), 25-30 minutes. Invert onto a largeplate. Slide into skillet browned side up.Cook until golden on bottom and cookedthrough, about 4 minutes more.
Serve warmor at room temperature.
Cut into wedges.DO AHEAD: Tortilla can be made 2 hoursahead.