Begin by peeling the potatoes or rutabaga and the onion. Slice the potatoes or rutabaga into thin rounds, aiming for about 1/8 inch thick. Dice the onion into small pieces.

In a small skillet or frying pan, heat the olive oil over medium heat. Add the sliced potatoes or rutabaga along with the diced onion. Cook for approximately 20 minutes, stirring occasionally to prevent sticking and ensure even cooking, until the potatoes or rutabaga are tender when pierced with a knife.

In a medium bowl, whisk the eggs and season with salt and black pepper. Incorporate the cooked potatoes or rutabaga and onion into the egg mixture, stirring well to combine.

If the skillet is dry, add a little more oil. Pour the egg mixture into the skillet and cook over medium-low heat until the edges are set and the center is mostly firm. Turn on the broiler and place the pan under it until the top is fully set, but not overly browned.

Let the tortilla rest briefly before loosening it from the pan. Place a plate over the pan and carefully flip it to transfer the tortilla onto the plate. Serve warm or at room temperature, cut into slices or cubes.

Enjoy your Spanish tortilla!