1. Peel, quarter, and cut the potatoes into one eight inch slices. Toss 4 tablespoons oil, potatoes, onion, ½ teaspoon salt, pepper, and red pepper flakes in a large bowl until potato slices are thoroughly separated and coated in oil. Heat 2 tablespoons more oil in 10-inch non-stick skillet over medium-high heat until shimmering. Reduce heat to medium-low, add potato mixture to skillet, and set bowl aside without washing. Cover and cook, stirring with heatproof rubber spatula every 5 minutes, until potatoes offer no resistance when poked with a paring knife, 22 to 28 minutes (some potato slices may break into smaller pieces).​

2. Meanwhile, whisk eggs and remaining ½ teaspoon salt in reserved bowl until just combined. Using a rubber spatula, fold hot potato mixture into eggs until combined, making sure to scrape all of potato mixture out of skillet. Return skillet to medium-high heat, add remaining 1 teaspoon of oil, and hat until just beginning to smoke. Add egg-potato mixture and cook, shaking pan and folding mixture constantly for 15 seconds. Smooth top of mixture with rubber spatula. Reduce heat to medium, cover, and cook, gently shaking pan every 30 seconds until bottom is golden brown and top is lightly set, about 2 minutes.

3. Using a rubber spatula, loosen tortilla from pan, shaking back and forth until tortilla slides around freely in pan. Slide tortilla onto a large plate, invert onto second large plate, and slide, browned side up, back into skillet. Tuck edges of tortilla into skillet with rubber spatula. Return pan to medium heat and continue to cook, gently shaking pan every 30 seconds, until second side is golden brown, about 2 minutes longer. Slide tortilla onto cutting board or serving plat and let cool for at least 15 minutes.