Peel and thinly slice 500 grams of potatoes and 1 large onion. Set aside.
In a large non-stick frying pan, heat 4 tablespoons of olive oil over medium heat. Add the sliced potatoes and onions, and cook for 20-25 minutes, stirring occasionally, until they are soft and slightly golden.
In a large bowl, beat 6 eggs and season with salt and pepper to taste. Add the cooked potatoes and onions to the beaten eggs, and mix gently to combine.
In the same frying pan, add 2 tablespoons of olive oil and heat over medium heat. Pour the egg-potato mixture into the pan, spreading it evenly.
Cook the tortilla for 5-7 minutes on one side, until the bottom is golden brown. Carefully flip the tortilla using a large plate or a flat lid, and cook the other side for another 5-7 minutes, until golden brown and cooked through.
Remove the tortilla from the pan and let it cool for a few minutes. Slice into wedges and serve warm or at room temperature.