The Spanish-style tortilla has more in common with a quiche or an omelet than it does with a Mexican tortilla. This is not something you use to wrap other ingredients. Rather, it can be a meal on its own.
¼ cup of good quality extra virgin olive oil
3 medium-sized red potatoes, peeled or unpeeled, cut into ½-inch chunks
1 medium onion, diced
Salt and pepper
6 eggs
In a 10-inch nonstick skillet, heat the olive oil over medium. When the oil is shimmering, add the chopped potatoes and onions. Sprinkle with salt and pepper. Cook, stirring occasionally, until the potatoes are cooked. Whisk the eggs and pour them over the potatoes and onions in the pan. Let them cook without stirring until the eggs are nearly set. To help the process, you can gently lift the edges of the cooked egg so that uncooked egg slips under that section.
When nearly set, slide the whole pie-shaped mixture onto a plate or the lid of the pan. Then invert the pan on top of the eggs and flip the pan, the eggs, and the plate so that the uncooked side of the tortilla is now on the heat. Cook all the way through.
To serve, cut into wedges.