This recipe comes from my cookbook Repertoire, out April 2018 (Little, Brown and Company).
When I make a tortilla, I cook it until it’s set on the bottom, then pop it under the broiler to brown and set the top. No flipping required. It may not be traditional, but you won’t care about tradition if your tortilla ends up on the floor, right? My nouveau technique requires you to use a pan that can go from stovetop to broiler. —Jessica Battilana
With a mandoline or sharp knife, slice the potatoes into 1/4-inch-thick coins. Heat the olive oil in a 9-inch cast-iron frying pan over medium heat. Add the potatoes and onion and stir gently, then push down beneath the oil (the
pan will be a bit crowded at first, but as the onions cook, the volume will decrease). Reduce the heat so the oil is bubbling gently and cook until the potatoes and onions are tender but not browned, about 10 minutes. Drain into a colander set over a heatproof bowl and reserve the oil.
In a large bowl, beat the eggs until well mixed. Season generously with salt and pepper and gently stir in the potato-onion mixture. In the same frying pan you used to cook the potatoes and onions, heat 1 tablespoon of the reserved oil over medium heat. Arrange an oven rack about 3 inches from the broiler and preheat the broiler.
Pour the egg-potato mixture into the frying pan, reduce the heat to medium-low, and cook until the edges of the tortilla are set and the underside is golden brown (peek to ensure the bottom isn’t browning too quickly), about 5 minutes, periodically running a rubber spatula around the inner edges of the pan to prevent
the eggs from sticking; the top will still be runny.
Transfer the pan to the oven and broil until the top is puffed, golden brown, and set, about 3 minutes. Remove from the oven and run the spatula around the inner edges of the pan to loosen the tortilla. Invert a large plate over the pan and, in one motion, flip the pan over so the tortilla is on the plate. Let cool slightly, then sprinkle with flaky salt and serve with aioli (see the full article for a "Fancy Mayonnaise" hack).