Step 1

Heat 1 Tbsp. oil in a large skillet over medium heat. Add onion, season with salt, and cook, stirring occasionally, until softened and dark brown, 35–40 minutes. Let cool slightly.

Step 2

Meanwhile, heat potatoes and remaining 2 cups oil in a medium saucepan over medium-high heat until oil around potatoes begins to bubble; reduce heat to medium and cook until potatoes are tender but have not taken on any color, 10–12 minutes. Drain potatoes, reserving oil. Season potatoes with salt and let cool slightly.

Step 3

Combine eggs, onion, potatoes, and ¼ cup reserved potato cooking oil in a large bowl and gently beat with a fork.

Step 4

Heat 3 Tbsp. reserved potato cooking oil in a 10” nonstick skillet over medium heat (reserve remaining oil for another use). Add egg mixture and cook, lifting at edge and tilting skillet to let uncooked egg run underneath, until bottom and edge of tortilla are set but center is still wet.

Step 5

Set a large plate on top of skillet. Swiftly invert tortilla onto plate, then slide back into skillet, cooked side up. Cook until center is just set, about 2 minutes longer. Cut into wedges.