Toss 4 tablespoons oil, potatoes, onion, ½ teaspoon salt, and pepper in large bowl until potato slices are thoroughly separated and coated in oil.
Heat 2 tablespoons oil in 10-inch nonstick skillet over medium-high heat until shimmering.
Reduce heat to medium-low, add potato mixture in skillet, and set bowl aside without washing.
Cover and cook, stirring with rubber spatula every 5 minutes, until potatoes offer no resistance when poked with tip of paring knife, 22 to 28 minutes (it's OK if some potato slices break into smaller pieces).
Meanwhile, whisk eggs and remaining ½ teaspoon salt in reserved bowl until just combined.
Using rubber spatula, fold hot potato mixture, red peppers, and peas into eggs until combined, making sure to scrape all of potato mixture out of skillet.
Return skillet to medium-high heat, add remaining teaspoon oil, and heat 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.
Using rubber spatula, loosen tortilla from pan, shaking back and forth until tortilla slides around freely in pan.
Slide tortilla onto large plate.
Invert tortilla onto second large plate and slide it 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 plate and allow to cool at least 15 minutes.
Cut tortilla into cubes or wedges and serve with aioli, if desired.
June 17, 2009 Made with Bea. I one-and-a-halved the recipe and we ate the whole thing for dinner. Everyone loved it. From Cook’s Illustrated, July & August 2009.