Potato and Onion Spanish Tortilla

Heat 1 tablespoon of the butter in a frying pan and cook the potatoes, turning frequently, for 10 minutes. Add the onions, and season with salt and pepper. Continue to cook gently for a further 10 minutes, until the potatoes and onions are tender. Beat the eggs in a large bowl. Add the potatoes and onions as well as any of the optional ingredients you are including in the dish. Mix gently and leave to stand for 10 minutes. Wipe out the frying pan with a paper towel and heat the remaining butter in it. Pour the egg mixture into the pan and spread it into an even layer. Cover and cook over very low heat for 20 minutes, until the eggs are just set. Cut into wedges and serve.