Cook the potatoes and onions
Heat 1/4 cup olive oil in a skillet over medium heat. Add the sliced potatoes and chopped onion. Cook gently, stirring occasionally, until the potatoes are fork-tender and evenly cooked.
Drain the mixture
Transfer the cooked potatoes and onions to a bowl and carefully discard any excess oil.
Prepare the egg mixture
In a large bowl, whisk the eggs. Add the cooked potatoes and onions, then season with salt and black pepper. Stir gently to combine.
Cook the tortilla
Heat the remaining 2 tablespoons olive oil in the same skillet. Pour in the potato and egg mixture, spreading it evenly with a spatula and smoothing the top.
Cook the first side
Cook over medium heat for 8–10 minutes until the bottom is golden brown and set.
Flip the tortilla
Place a large round plate over the skillet. Firmly hold the plate and carefully flip the tortilla onto it.
Cook the second side
Return the skillet to the heat and gently slide the tortilla back into the pan. Cook for another 8–10 minutes until golden brown. For a softer, slightly runnier center, cook for 6–8 minutes instead.
Serve
Once both sides are golden and cooked to your liking, transfer to a serving plate. Slice and serve warm, room temperature, or chilled.