Recipe adapted from Aran Goyoaga and her blog, Cannelle et Vanille.
In a 12-inch nonstick skillet, heat oil over medium-high. Add onion and cook until beginning to soften, 2 minutes. Add potatoes and 1 teaspoon salt and cook 2 minutes. Reduce heat to medium and cook until potatoes are tender but not brown, 25 minutes, stirring often.
Meanwhile, in a large bowl, whisk together eggs and 1/2 teaspoon salt. With a slotted spoon, add potato mixture to eggs and stir to combine. Pour off oil from skillet; reserve 2 teaspoons.
In an 8-inch nonstick skillet, heat 1 teaspoon reserved oil over medium. Add half the egg and potato mixture and, with a wooden spoon, stir center until eggs begin to set. Use spoon to lightly press edges of tortilla inward and continue to cook, undisturbed, about 2 minutes. Remove pan from heat, cover with a plate, and invert tortilla onto plate. Gently slide tortilla back into pan and cook until set in middle, about 5 minutes. Slide or invert tortilla onto a clean plate, wipe pan clean with a paper towel, and repeat with remaining oil and egg mixture. Let tortillas sit 5 minutes before cutting each into 6 wedges. Serve warm or at room temperature.