In a medium-sized skillet (8-9" or as large as you can go while being able to cover the top), heat enough olive oil to cover the bottom of the pan over medium heat. Layer the potatoes and onion in the oil, seasoning each layer with salt and pepper as you go. Cook the potatoes and onions until soft, but not browned, about 10-15 minutes.Once the potato mixture is softened, drain a little of the oil out (leaving about a tablespoon in the pan) and pour in the eggs (enough to cover the potatoes). Let the eggs cook to set. Once the bottom is lightly browned and the middle starts setting, place your plate over the top of the skillet. Carefully flip the tortilla onto the plate and then slide it back into the pan, uncooked side down. Cook a little longer until the bottom is also lightly browned.

Slide the tortilla onto a clean plate and let cool. Tortillas are traditionally served at room temperature, so this can be made ahead (but no longer than a couple of hours as the potatoes will start to turn brown). Slice into triangles and serve with salad or if serving as an appetizer, cut into small, bite-sized squares.