1. Crack the eggs into a large mixing bowl, add a generous pinch of salt, and whisk vigorously until light and airy. Set aside for now.

2. While the eggs rest, heat a good amount of oil in a 10-inch nonstick or well-seasoned carbon steel skillet over medium-high heat until it begins to shimmer. Add the sliced potatoes and onions. You’re looking for a gentle sizzle—adjust the heat as needed to avoid browning. Let them cook, stirring occasionally, until both the potatoes and onions are soft and buttery in texture, about 25 minutes.

3. Place a fine-mesh strainer over a heatproof bowl and carefully drain the mixture, saving the flavorful oil for later use.

4. Transfer the drained potatoes and onions to another bowl and season them well with salt. Re-whisk the eggs to bring back some froth, then stir in the potato-onion mixture until evenly coated. Let this sit for about 5 minutes so the flavors can meld.

5. Wipe the skillet clean. Pour in about 3 tablespoons of the reserved oil and heat over medium-high. When hot, pour in the egg and potato mixture. Quickly swirl and shake the skillet so the bottom cooks evenly. Let it cook undisturbed until the edges begin to set, about 3 minutes. Use a heatproof spatula to gently tuck in the sides, forming a rounded shape. Continue cooking, adjusting the heat to avoid scorching, for another few minutes until the tortilla is mostly set around the edges.

6. To flip: place a large plate or flat lid over the skillet, then carefully and quickly invert the tortilla onto it (you may want to use a towel to protect your hand). Add another tablespoon of the reserved oil to the pan, then slide the tortilla back in to cook the other side. Let it cook for another 2 minutes, then use the spatula again to shape the edges. Cook for another couple of minutes until the bottom is lightly golden but the center still feels soft to the touch.

7. If desired, flip the tortilla two or three more times during the final minutes to ensure even cooking and a well-formed shape.

8. When done, slide or flip the tortilla onto a clean plate. Let it rest at least 5 minutes before serving. This dish can be served warm or at room temperature. Slice into wedges for a meal or into small cubes for snacks. It keeps well in the fridge for up to 3 days—just let it come to room temp before eating. Any leftover cooking oil can be reused; it will be rich in flavor from the potatoes and onions.

adapted from: https://www.seriouseats.com/tortilla-espanola-spanish-potato-omelette-recipe